Attempt to build OpenMM 7.4.2rc

[jaimergp]

Taking over attempts at:

• [openmm] 7.4.2 rc conda-recipes-cuda#23
• [openmm] 7.4.2 rc alternative build attempt conda-recipes-cuda#24

If this works we might consider merging to a releases/* branch that will receive special treatment for the channel and label uploads.

[jaimergp]

@jchodera 7.4.2rc builds here with no issue. If you agree, my plan would be:

1. Adapt the current infrastructure to deal with releases/* branches. These will run unscheduled (only on push) and upload to omnia directly (but NOT using the main - this is only meant for final releases). Packages from this branch will have labels prefixed with rc. This shouldn't take too much effort, but must be done carefully. I will work on a dummy package to test it.
2. Create a new branch from master, named releases/openmm-7.4.2-rc.
3. Merge this PR to releases/openmm-7.4.2-rc
4. Review packages with the rc* label.