CI: Fix the script to install dependencies on Windows

[seisman]

The CI workflow suddenly started to fail. The error message is:

D:\a\gmt\gmt>L "C:\Miniconda\condabin\..\Scripts\conda.exe"   update -n base -c conda-forge conda --solver libmamba 
'L' is not recognized as an internal or external command,
operable program or batch file.

but we are actually running

'C:\Miniconda\condabin\conda.bat' update -n base -c conda-forge conda --solver libmamba

in the script.

So, there must be something wrong in the conda.bat script. This is likely a bug in GitHub-hosted agent and we can do nothing.

This PR fixes it by calling the conda.exe command directly.