Set -Crelocation-model=static in RUSTFLAGS

In d3b9c05, we switched our test framework to the built-in x86_64-unknown-none target. This required a `-Crelocation-model=static` flag to work with bootloader v0.9, which we set using a `build.rustflags` config key.

In 2eb838d, we set a `RUSTFLAGS` env variable on CI to deny warnings.

Unfortunately, the above two commits conflict because the `RUSTFLAGS` env variable takes precedence over the `build.rustflags` key. This commit fixes this by setting an explicit `RUSTFLAGS` value for the bootloader test job that contains both flags.

Author: phil-opp

.github/workflows/build.yml

@@ -83,6 +83,9 @@ jobs:
     runs-on: ${{ matrix.platform }}
     timeout-minutes: 15
 
+    env:
+      RUSTFLAGS: -Crelocation-model=static -Dwarnings
+
     steps:
       - name: "Checkout Repository"
         uses: actions/checkout@v4