Artifact and GMT 6.5.0 ghostscript mismatch (with suggested fixes)

GMT.jl rules!

When running GMT.jl 1.21.1 while having a build of GMT 6.5.0 one gets an error:
`gs: Interpreter revision (9550) does not match gs_init.ps revision (10021)`

The error is due to the fact that:
- GMT.jl 1.21.1 uses artifact Ghostscript version 9.55.0
- GMT 6.5.0 (kindly) ships with Ghostscript version 10.02.1 which now asks to set environmental variable GS_LIB (looks like this changed from gs 9->10)
- Ghostscript requires any gs_init.ps found to have listed version (in this case, 10021, on first line) to be the same as the executable (in this case, 9550)

Here are 2 suggested fixes:
1. Requires a patch:
- `unset GS_LIB` in psconvert System call (e.g., System call [unset GS_LIB; '/xxx/.julia/artifacts/xxx/bin/gs ...'])
2. Can be done by user:
- delete the environmental variable at startup (e.g., `delete!(ENV, "GS_LIB")` in ~/.julia/config/startup.jl)