Env must be set on job level

This is likely the issue for cache to not include correct Gemfile and
reuse set of conflicting gem versions.

Author: mostlyobvious

.github/workflows/protobuf_nested_struct_mutate.yml

@@ -13,6 +13,8 @@ jobs:
   mutate:
     runs-on: ubuntu-latest
     timeout-minutes: 120
+    env:
+      RUBYOPT: "--enable-frozen-string-literal"
     steps:
       - uses: actions/checkout@v4
         with:
@@ -23,5 +25,3 @@ jobs:
           ruby-version: ruby-3.4
           bundler-cache: true
       - run: bin/mutant run
-        env:
-          RUBYOPT: "--enable-frozen-string-literal"

.github/workflows/protobuf_nested_struct_test.yml

@@ -18,6 +18,9 @@ jobs:
       matrix:
         ruby: [ruby-3.4, ruby-3.3, ruby-3.2]
         gemfile: [Gemfile, Gemfile.google_protobuf_3_25]
+    env:
+      BUNDLE_GEMFILE: "${{ matrix.gemfile }}"
+      RUBYOPT: "--enable-frozen-string-literal"
     steps:
       - uses: actions/checkout@v4
         with:
@@ -28,6 +31,3 @@ jobs:
           ruby-version: "${{ matrix.ruby }}"
           bundler-cache: true
       - run: bin/rspec
-        env:
-          BUNDLE_GEMFILE: "${{ matrix.gemfile }}"
-          RUBYOPT: "--enable-frozen-string-literal"