Fixes MacOS nightly; Introduces nightly mdbook build (#380)

Nightly MacOS test runs were failing due to the updated integration test
structure recently introduced to the project. Part of these changes
addresses that situation.

The other part of the changes introduces an mdbook nightly build. This
does not publish the documentation, it only checks to make sure that
mdbook+plugins can install and build the documentation properly. I
believe it was necessary because the only time documentation is built is
when a release happens, and if documentation generation fails for
whatever reason, we find out only after a release has been published.
The hope is that this should catch any errors before we release.

NOTE: This PR wants to be merged into tcdi/plrust:main directly, as it
is only CI changes and the nightly updates can be applied immediately
instead of waiting for another plrust release.

Author: BradyBonnette

.github/workflows/nightly.yml

@@ -277,7 +277,7 @@ jobs:
         psql -h 127.0.0.1 -U postgres -c 'SELECT * FROM plrust.one()'
 
   plrust_macos_x86_64:
-    name: macos x86_64 tests
+    name: (nightly) MacOS x86_64 tests
     runs-on: ${{ matrix.os }}
     if: "!contains(github.event.head_commit.message, 'nogha')"
 
@@ -357,13 +357,74 @@ jobs:
     - name: Install plrustc
       run: cd plrustc && ./build.sh && cp ../build/bin/plrustc ~/.cargo/bin
 
-    - name: Run 'cargo pgrx init' against system-level ${{ matrix.version }}
+    - name: Run cargo pgrx init
       run: cargo pgrx init --pg$PG_VER $(which pg_config)
 
-    - name: Test PL/rust as "untrusted"
-      if: matrix.target == 'host'
-      run: cargo test --all --features "pg$PG_VER" --no-default-features
+    - name: Install PL/Rust as "trusted"
+      if: matrix.target == 'postgrestd'
+      run: cd plrust && STD_TARGETS="x86_64-apple-darwin-postgres" ./build && echo "\q" | cargo pgrx run "pg$PG_VER" --features "trusted"
+
+    - name: Test PL/Rust package as "trusted"
+      if: matrix.target == 'postgrestd'
+      run: cd plrust && cargo test --no-default-features --features "pg$PG_VER trusted"
 
-    - name: Test PL/rust as "trusted" (inc. postgrestd)
+    - name: Run PL/Rust integration tests as "trusted"
       if: matrix.target == 'postgrestd'
-      run: cd plrust && STD_TARGETS="x86_64-apple-darwin-postgres" ./build && cargo test --verbose --no-default-features --features "pg$PG_VER trusted"
+      run: cd plrust && echo "\q" | cargo pgrx run "pg$PG_VER" --features "trusted" && cd ../plrust-tests && cargo test --no-default-features --features "pg$PG_VER trusted"
+
+    - name: Install PL/Rust as "untrusted"
+      if: matrix.target == 'host'
+      run: cd plrust && STD_TARGETS="x86_64-apple-darwin-postgres" ./build && echo "\q" | cargo pgrx run "pg$PG_VER"
+
+    - name: Test PL/Rust package as "untrusted"
+      if: matrix.target == 'host'
+      run: cd plrust && cargo test --no-default-features --features "pg$PG_VER"
+
+    - name: Run PL/Rust integration tests as "untrusted"
+      if: matrix.target == 'host'
+      run: cd plrust && echo "\q" | cargo pgrx run "pg$PG_VER" && cd ../plrust-tests && cargo test --no-default-features --features "pg$PG_VER"
+
+  test_mdbook:
+    name: (nightly) Test that mdbook can process the documentation correctly
+    runs-on: [ubuntu-latest]
+    defaults:
+      run:
+        shell: bash
+
+    strategy:
+      fail-fast: false
+
+    env:
+      CI: true
+
+    steps:
+    # If this workflow is being called from a schedule/cron job, then let's
+    # force the "develop" branch. Otherwise, use whatever is passed in via
+    # GITHUB_HEAD_REF. The result of this will be used below in the
+    # actions/checkout@vX step. Note that at the time of this writing, Github
+    # Actions does not allow us to specify which branch to run a schedule from
+    # (it always runs from the default branch, which in this case is master).
+    - name: Set up correct branch environment variable
+      run: |
+        if [ $GITHUB_EVENT_NAME == "schedule" ]; then
+          echo "Running via schedule, so using branch develop"
+          echo "NIGHTLY_BUILD_REF=develop" >> $GITHUB_ENV
+        else
+          echo "Not running via schedule, so using branch $GITHUB_HEAD_REF"
+          echo "NIGHTLY_BUILD_REF=$GITHUB_HEAD_REF" >> $GITHUB_ENV
+        fi
+
+    - uses: actions/checkout@v3
+      with:
+        ref: ${{ env.NIGHTLY_BUILD_REF }}
+
+    - name: Install Rust
+      run: |
+        curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+        echo "PATH=$HOME/.cargo/bin:$PATH" >> $GITHUB_ENV
+
+    - name: Install mdbook and other preprocessors
+      run: cargo install --locked mdbook-variables mdbook
+
+    - name: Build documentation
+      run: cd doc && mdbook build