Use --release-channel=stable by default on releases

> Release tarballs should be compilable with just basic ./configure ;
> make ; sudo make install without having to pass special flags to
> configure. This is the case of the --release-channel option, that must
> be changed in the releases.

This commit detects the presence of .git, as it happens on other parts
of `configure` to assume it is a tarball. Then it changes the default
value stored, before parsing the arguments, while still allowing it to
be overriden before any action verifying the flag is done.

Closes #28322

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