Windows, Bazel client: respect envvars
Windows, Bazel client: respect envvars

Respect HOME for the home directory, and USER and
USERNAME for the user name, before trying to fetch
these values from API functions.

Respecting the envvars allows overriding their
value, which is not only more power for the user,
but also an escape hatch to work around bugs like
https://github.com/bazelbuild/bazel/issues/7819#issuecomment-533050947

This is a step towards fixing https://github.com/bazelbuild/bazel/issues/7819

Closes #9430.

PiperOrigin-RevId: 270909221

dslomov/bazel,davidzchen/bazel,dslomov/bazel,dslomov/bazel,aehlig/bazel,werkt/bazel,akira-baruah/bazel,aehlig/bazel,davidzchen/bazel,cushon/bazel,dslomov/bazel,aehlig/bazel,safarmer/bazel,dslomov/bazel,ButterflyNetwork/bazel,perezd/bazel,davidzchen/bazel,twitter-forks/bazel,katre/bazel,davidzchen/bazel,meteorcloudy/bazel,twitter-forks/bazel,meteorcloudy/bazel,twitter-forks/bazel,ulfjack/bazel,bazelbuild/bazel,perezd/bazel,katre/bazel,cushon/bazel,ulfjack/bazel,bazelbuild/bazel,perezd/bazel,ButterflyNetwork/bazel,werkt/bazel,twitter-forks/bazel,safarmer/bazel,katre/bazel,bazelbuild/bazel,twitter-forks/bazel,cushon/bazel,aehlig/bazel,akira-baruah/bazel,aehlig/bazel,bazelbuild/bazel,meteorcloudy/bazel,perezd/bazel,aehlig/bazel,dslomov/bazel-windows,meteorcloudy/bazel,cushon/bazel,katre/bazel,dslomov/bazel,perezd/bazel,dslomov/bazel-windows,ulfjack/bazel,davidzchen/bazel,davidzchen/bazel,cushon/bazel,dslomov/bazel-windows,ButterflyNetwork/bazel,davidzchen/bazel,safarmer/bazel,twitter-forks/bazel,aehlig/bazel,akira-baruah/bazel,meteorcloudy/bazel,perezd/bazel,safarmer/bazel,dslomov/bazel-windows,safarmer/bazel,werkt/bazel,perezd/bazel,dslomov/bazel-windows,ButterflyNetwork/bazel,meteorcloudy/bazel,ulfjack/bazel,werkt/bazel,ButterflyNetwork/bazel,akira-baruah/bazel,ButterflyNetwork/bazel,katre/bazel,ulfjack/bazel,bazelbuild/bazel,katre/bazel,meteorcloudy/bazel,twitter-forks/bazel,dslomov/bazel-windows,werkt/bazel,bazelbuild/bazel,ulfjack/bazel,ulfjack/bazel,cushon/bazel,akira-baruah/bazel,dslomov/bazel,akira-baruah/bazel,werkt/bazel,safarmer/bazel