GHA Windows: conda dep conflicts

[bschilder]

echoR can't build bc of some dependency conflicts.

https://github.com/RajLabMSSM/echoconda/runs/8194389596?check_suite_focus=true#step:21:647

[bschilder]

Resolved. Windows is excluded from the echoconda CI matrix (htslib/tabix not available on Windows conda-forge). The rworkflows.yml comment documents this: "Windows excluded: htslib/tabix not available on Windows conda-forge".