Use standard `NETRC` environment variable name for netrc credential file

The previously used environment variable name required the downstream
tool (release/make-release/automatic-branch-cut) to set HOME and
symlink the netrc file. If we instead use the standard NETRC
environment variable name, the file should be picked up by Python's
requests library and there will be no need for special handling in the
branch cut script.

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Related MRs:

• !68 (merged)
• !67 (merged) (This MR)
• master