CI: Use latest rubygems on github actions

In order to solve

    undefined method `parse' for Psych::Parser

errors while installing sass-embedded.

Author: tvdeyen

.github/workflows/build_test.yml

@@ -117,6 +117,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+          rubygems: "latest"
       - name: Restore apt cache
         id: apt-cache
         uses: actions/cache@v4

.github/workflows/lint.yml

@@ -17,6 +17,7 @@ jobs:
         with:
           ruby-version: "3.0"
           bundler-cache: true
+          rubygems: "latest"
       - name: Lint Ruby files
         run: bundle exec standardrb
   ESLint: