build: fix vcbuild format-md install and exit

[vedchaudhari]

With vcbuild format-md, the shortcut that skips building went
straight to format-md and skipped lint-md-build. This meant the
Markdown dependencies were not installed on a fresh clone, causing
the command to print the source tarball message without formatting
anything.

Also, format-md did not exit after formatting, so execution fell
through to no-lint and printed the same misleading message.

Make the format-md shortcut go through lint-md-build first and
exit after formatting.

I ran vcbuild format-md after deleting tools\lint-md\node_modules to verify the Markdown dependencies are installed and formatting works on a fresh checkout. It completed without the source tarball message and exited with code 0. I ran vcbuild format-md again with the dependencies already installed, and also checked that vcbuild lint-md and vcbuild lint still work. No files under doc\ were changed. There are no automated tests for vcbuild.bat, so I tested the fix manually.

Refs: #66467

Assisted-by: claude:opus-5.5

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