ci: only save cache on main

Author: Firestar99

.github/workflows/ci.yaml

@@ -47,6 +47,7 @@ jobs:
         with:
           add-job-id-key: "false"
           shared-key: "base"
+          save-if: ${{ github.ref_name == 'main' }}
       # Fetch dependencies in a separate step to clearly show how long each part
       # of the testing takes
       - name: cargo fetch --locked
@@ -87,6 +88,7 @@ jobs:
       # * difftests depends on `spirv-builder` with no features, which excludes `rustc_codegen_spirv` entirely.
       #   The individual difftest crates depend on it and run the spirv compile, never the test runner itself.
       - name: prep cache for other jobs
+        if: github.ref_name == 'main'
         run: |
           cargo build -p compiletests --release --no-default-features --features "use-installed-tools"
           cargo build -p difftests --release --no-default-features --features "use-installed-tools"
@@ -114,6 +116,7 @@ jobs:
         with:
           add-job-id-key: "false"
           shared-key: "android"
+          save-if: ${{ github.ref_name == 'main' }}
       - name: cargo fetch --locked
         run: cargo fetch --locked --target ${{ matrix.target }}