consider $RAMBLE_PYTHON environment variable in ramble script to pick python command to use

[rfbgo]

Based on #531 from @boegel:

On some systems multiple python* commands are available, like python3.9 on a RHEL8 system next to the standard system Python python3 which is equivalent to python3.6.

If ramble picks up on an environment variable like $RAMBLE_PYTHON, the user can control which python* command is used to run Ramble...

[rfbgo]

Fixed in #555