Change shebang line to #!/usr/bin/env python

[ZoeLeah]

Fix issue #5983 with actions that are executed remotely on the server, for example linux.service or linux.check_processes.
The following error message was returned: /opt/stackstorm/st2/bin/python: bad interpreter: No such file or directory
Change the shebang line #!/opt/stackstorm/st2/bin/python to #!/usr/bin/env python.
It was also changed #!/usr/bin/python to #!/usr/bin/env python to have it consistent

[CLAassistant]

All committers have signed the CLA.

[arm4b]

@ZoeLeah Thanks for the fix 👍 Appreciate the contribution.

The CI is happy, the PR just needs a better changelog, otherwise looks good!

[arm4b]

Because it's a bugfix, I'm thinking we can plan to include it in the upcoming v3.8.1 patch version.

[arm4b]

Looks good!