Check GAMS dir also in PATH on Windows

[soininen]

We used to check for GAMS directory in Windows registry only. Now we check the PATH environment variable too as with other systems.

Re spine-tools/Spine-Toolbox#1974

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass