Try to configure env at runtime

Author: saethlin

.github/workflows/ci.yml

@@ -18,7 +18,6 @@ jobs:
     env:
       RUST_BACKTRACE: 1
       HOST_TARGET: ${{ matrix.host_target }}
-      MIRIFLAGS: -Zmiri-tag-gc=${{ matrix.os == 'ubuntu-latest' }}
     strategy:
       matrix:
         build: [linux64, macos, win32]
@@ -35,6 +34,12 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - name: Configure
+        run: echo "MIRIFLAGS=-Zmiri-tag-gc=$([[ ${{ matrix.os }} == \"ubuntu.latest\" ]]; echo $?)" >> $GITHUB_ENV
+
+      - name: Test
+        run: echo $MIRIFLAGS
+
       # We install gnu-tar because BSD tar is buggy on macOS builders of GHA.
       # See <https://github.com/actions/cache/issues/403>.
       - name: Install GNU tar