Add --exists-action option to git checkouts for pip

[ocelotl]

Fixes #3879

[tammy-baylis-swi]

Makes sense with docs saying the flag will set default action for What to do? (s)witch, (i)gnore, (w)ipe, (b)ackup py38-opentelemetry-getting-started: exit 2 (1.12 seconds). Though could the lines be removed altogether if something above is already installing these?

[xrmx]

We can improve it later but let's fix the builds :)