Use locked Bundler version for composed bundle #10979

Job	Run time
lint_ruby	19s
lint_node	26s
Verify Jekyll website builds	11s
build_node (ubuntu-latest)	1m 16s
build_node (macos-latest)	1m 11s
Ruby 3.1 on ubuntu-latest	15m 42s
build_node (windows-latest)	7m 48s
Ruby 3.2 on ubuntu-latest	13m 30s
Ruby 3.3 on ubuntu-latest	12m 2s
Ruby 3.1 on macos-latest	11m 32s
Ruby 3.2 on macos-latest	8m 16s
Ruby 3.3 on macos-latest	6m 44s
Ruby 3.1 on windows-latest	22m 16s
Ruby 3.2 on windows-latest	22m 8s
Ruby 3.3 on windows-latest	24m 30s
	2h 27m 51s

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