* Test that setup_run_environment changes to CC/CXX/FC/F77 are dropped in build env
* compilers set in run env shouldn't impact build
Adds `drop` to EnvironmentModifications courtesy of @haampie, and uses
it to clear modifications of CC, CXX, F77 and FC made by
`setup_{,dependent_}run_environment` routines when producing an
environment in BUILD context.
* comment / style
* comment
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Co-authored-by: Tom Scogland <scogland1@llnl.gov>
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