Add new `brioche-packer update-source` subcommand #54

Job	Run time
Run tests (ubuntu-22.04)	1m 3s
Build artifacts (x86_64-linux, ubuntu-22.04, x86_64-unknown-linux-gnu, x86_64-unknown-linux-musl,...	1m 39s
Run tests (macos-14)	1m 8s
Run checks	24s
Build artifacts (x86_64-macos, macos-14, x86_64-apple-darwin, x86_64-apple-darwin, brioche-packed...	2m 12s
Build artifacts (aarch64-macos, macos-14, aarch64-apple-darwin, aarch64-apple-darwin, brioche-pac...	1m 51s
Push artifacts	15s
	8m 32s

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