Bug fixes from script runs

[truth-quark]

Quick catalogue of bugs from Lan-Wei's test runs:

• PyGamma shim env var for GAMMA install location needs to be set somewhere outside insar.env
• Convert all args to strs here for creating the Gamma command string (& add some tests) https://github.com/GeoscienceAustralia/gamma_insar/blob/pygamma_workflow/insar/py_gamma_ga.py#L74
• The Gamma shim only works with 1 arg

[truth-quark]

For fixing the shim bug with passing args to pygamma: https://github.com/GeoscienceAustralia/gamma_insar/blob/pygamma_workflow/insar/py_gamma_ga.py#L80

subprocess.run() needs separate args. To fix, replace with:

cmd_list = [cmd]
cmd_list.extend(str(a) for a in args)