ci: Enable `llvm-link-static` feature for bpf-linker

Rust CI does not provide dynamic libLLVM tarballs for macOS—only static
ones. Since recent versions of bpf-linker require explicit linkage
configuration for libLLVM, enable the `llvm-link-static` feature to
ensure correct static linking.

Make sure bpf-linker's build.rs sees the downloaded LLVM by adding it
to PATH. On macOS, set the `{CXXSTDLIB,ZLIB}_PATH` variables to point
to the brew prefixes with static libraries it needs.

.github/workflows/ci.yml

@@ -279,6 +279,10 @@ jobs:
           brew bundle
           echo $(brew --prefix curl)/bin >> $GITHUB_PATH
           echo $(brew --prefix llvm)/bin >> $GITHUB_PATH
+          # Point bpf-linker's build.rs to the brew prefixes with static
+          # libraries it needs.
+          echo "CXXSTDLIB_PATH=$(brew --prefix llvm)/lib/c++" >> $GITHUB_ENV
+          echo "ZLIB_PATH=$(brew --prefix zlib)/lib" >> $GITHUB_ENV
 
           # https://github.com/actions/setup-python/issues/577
           find /usr/local/bin -type l -exec sh -c 'readlink -f "$1" \
@@ -324,7 +328,7 @@ jobs:
           esac
           case "${{ runner.os }}" in
             Linux)
-              target=${arch}-unknown-linux-gnu
+              target=${arch}-unknown-linux-musl
               ;;
             macOS)
               target=${arch}-apple-darwin
@@ -341,8 +345,13 @@ jobs:
       # NB: rustc doesn't ship libLLVM.so on macOS, so disable proxying (default feature). We also
       # --force so that bpf-linker gets always relinked against the latest LLVM downloaded above.
       #
+      # Add the downloaded LLVM to PATH for this build step.
+      #
       # Do this on all system (not just macOS) to avoid relying on rustc-provided libLLVM.so.
-      - run: cargo install --git https://github.com/aya-rs/bpf-linker.git bpf-linker --no-default-features --features llvm-21 --force
+      - run: |
+          PATH="/tmp/rustc-llvm/bin:$PATH" cargo install \
+            --git https://github.com/aya-rs/bpf-linker.git bpf-linker \
+            --no-default-features --features llvm-21,llvm-link-static --force
 
       - uses: actions/cache@v4
         with: