Allow switching between overrides

[juj]

Previously, if one used e.g. command

emsdk install --override-repository emscripten-main-64bit@https://github.com/juj/emscripten/tree/my_own_emscripten_branch emscripten-main-64bit

then the overridden installation would create a git clone from juj/emscripten, and that clone would be located under the default remote name origin.

This would prevent being able to switch between overrides on subsequent emsdk install commands. I.e. it would then not be possible to later do a default

emsdk install emscripten-main-64bit

command without an override, since the remote name origin would point to juj/emscripten instead of emscripten-core/emscripten.

This PR changes the naming scheme of git clones when --override-repository is used, to name the clones with remote names from the override, so in the above case, the clone would appear under remote name juj and not origin.

This enables flipping between multiple emsdk install commands in the same checkout, without needing to nuke the installed directory in between.

This enables me to run experiments like github.com/emscripten-core/emscripten/pull/25025 locally on my CI, without needing to merge them upstream to become available.

[juj]

@sbc100 would you like to review this? Or I'll go ahead and land, probably nobody else uses --override-repository than Unity.

[sbc100]

lgtm % comment

Indeed I think you are likely the only user of this code.