Fix shellcheck issues in workflows

`shellcheck` is reporting an SC2086 ("Double quote to prevent
globbing and word splitting") issue in the
`bump-unversioned-casks.yml` workflow and SC2016 ("Expressions don't
expand in single quotes, use double quotes for that") issues in the
`ci.yml` workflow.

The SC2016 issues seem to appear because the use of backticks around
the cask names (markdown syntax) can be interpreted as legacy
backticks (as used in shell code). The only way I could appease
`shellcheck` was to use double quotes and escape the backticks.

This passes `brew style` but I haven't tested it, so hopefully it
works as intended.

.github/workflows/bump-unversioned-casks.yml

@@ -61,7 +61,7 @@ jobs:
       - name: Generate cache ID
         id: cache-id
         run: |
-          echo "time=$(date -u +'%Y-%m-%dT%H:%M:%SZ')" >> $GITHUB_OUTPUT
+          echo "time=$(date -u +'%Y-%m-%dT%H:%M:%SZ')" >> "$GITHUB_OUTPUT"
 
       - name: Cache state
         uses: actions/cache@v4

.github/workflows/ci.yml

@@ -104,15 +104,15 @@ jobs:
           fi
 
           if ! rm /usr/local/bin/dotnet; then
-            echo '::warning::Removing `dotnet` symlink is no longer necessary.'
+            echo "::warning::Removing \`dotnet\` symlink is no longer necessary."
           fi
 
           if ! rm /usr/local/bin/pod; then
-            echo '::warning::Removing `cocoapods` symlink is no longer necessary.'
+            echo "::warning::Removing \`cocoapods\` symlink is no longer necessary."
           fi
 
           if ! rm /usr/local/bin/chromedriver; then
-            echo '::warning::Removing `chromedriver` symlink is no longer necessary.'
+            echo "::warning::Removing \`chromedriver\` symlink is no longer necessary."
           fi
 
           brew unlink python && brew link --overwrite python