chore: bump macos runner version (#27)

GitHub Action is sunsetting the macOS 10.15 Actions runner. It will stop working intermittently until being completely removed by 2022-8-30: https://github.blog/changelog/2022-07-20-github-actions-the-macos-10-15-actions-runner-image-is-being-deprecated-and-will-be-removed-by-8-30-22

.github/workflows/ci.yml

@@ -87,7 +87,7 @@ jobs:
               compiler: clang-12,  cxxstd: '03,11,14,17,20', os: ubuntu-20.04, stdlib: libc++, install: 'clang-12 libc++-12-dev libc++abi-12-dev' }
 
           # OSX, clang
-          - { compiler: clang,     cxxstd: '03,11,14,17,2a', os: macos-10.15, sanitize: yes }
+          - { compiler: clang,     cxxstd: '03,11,14,17,2a', os: macos-11, sanitize: yes }
 
           # Coverity Scan
           # requires two github secrets in repo to activate; see ci/github/coverity.sh
@@ -132,20 +132,23 @@ jobs:
             fi
             git config --global pack.threads 0
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         with:
           # For coverage builds fetch the whole history, else only 1 commit using a 'fake ternary'
           fetch-depth: ${{ matrix.coverage && '0' || '1' }}
 
       - name: Cache ccache
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         if: env.B2_USE_CCACHE
         with:
           path: ~/.ccache
-          key: ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}
+          key: ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}-${{github.sha}}
+          restore-keys: |
+            ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}-
+            ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}
 
       - name: Fetch Boost.CI
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           repository: boostorg/boost-ci
           ref: master