Support environment variable interpolation for junit_xml + quickstart_file config keys

[Mattwmaster58]

Expands on the goal of #7273
Exactly what the title says. The docs say this:

Some flags support user home directory and environment variable expansion. To refer to the user home directory, use ~ at the beginning of the path. To expand environment variables use $VARNAME or ${VARNAME}.

This would add quickstart_file and junit_xml to the flags that support environment variable expansion.

The only downside I see here is if someone decides they want their JUnit XML ouput file or quickstart file something weird like $file

[ilevkivskyi]

LGTM.