Fix building libservo with cargo build -p libservo (#35116)

* Fix building libservo with `cargo build -p libservo`

Signed-off-by: Delan Azabani <dazabani@igalia.com>

* Test the libservo build in CI

Signed-off-by: Delan Azabani <dazabani@igalia.com>

* Work around build issue on macOS (#34517)

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

.github/workflows/linux.yml

@@ -21,6 +21,10 @@ on:
         required: false
         default: false
         type: boolean
+      build-libservo:
+        required: false
+        default: false
+        type: boolean
       upload:
         required: false
         default: false
@@ -59,6 +63,10 @@ on:
         required: false
         default: false
         type: boolean
+      build-libservo:
+        required: false
+        default: false
+        type: boolean
       upload:
         required: false
         default: false
@@ -165,6 +173,9 @@ jobs:
           timeout_minutes: 20
           max_attempts: 2 # https://github.com/servo/servo/issues/30683
           command: ./mach test-unit --${{ inputs.profile }}
+      - name: Build libservo
+        if: ${{ inputs.build-libservo }}
+        run: cargo build -p libservo
       - name: Archive build timing
         uses: actions/upload-artifact@v4
         with: