docs: add bash lang to CONTRIBUTING.md code blocks #9166

Job	Run time
detect-changes	5s
Build Windows CLI binaries	0s
download-previous-rolldown-binaries	0s
${{ matrix.project.name }} E2E test (${{ matrix.os }})	0s
Build vite-plus packages (${{ matrix.os }})	0s
Notify on failure	0s
	5s

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