[develop] Update forecast task for RRFS_dev1 run.

[danielabdi-noaa]

DESCRIPTION OF CHANGES:

Update run_fcst to work with RRFS_dev1 workflow.
Note that there is no workflow so this PR can not be run independently until then.
However, it did successfully run for both spinup/production cycles in PR #540.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

TESTS CONDUCTED:

• hera.intel
  Successfully run both spinup and production forecasts as in PR [develop] Incorporate RRFS_dev1 workflow to SRW and import most tasks #540
• orion.intel
• cheyenne.intel
• cheyenne.gnu
• gaea.intel
• jet.intel
• wcoss2.intel
• NOAA Cloud (indicate which platform)
• Jenkins
• fundamental test suite
• comprehensive tests (specify which if a subset was used)

DEPENDENCIES:

• [develop] Add new RRFS variables such as NWGES and workflow control variables #647

DOCUMENTATION:

ISSUE:

• Modify forecast script to get correct background. #545

CHECKLIST

• My code follows the style guidelines in the Contributor's Guide
• I have performed a self-review of my own code using the Code Reviewer's Guide
• I have commented my code, particularly in hard-to-understand areas
• My changes need updates to the documentation. I have made corresponding changes to the documentation
• My changes do not require updates to the documentation (explain).
• My changes generate no new warnings
• New and existing tests pass with my changes
• Any dependent changes have been merged and published

LABELS (optional):

A Code Manager needs to add the following labels to this PR:

• Work In Progress
• bug
• enhancement
• documentation
• release
• high priority
• run_ci
• run_we2e_fundamental_tests
• run_we2e_comprehensive_tests
• Needs Cheyenne test
• Needs Jet test
• Needs Hera test
• Needs Orion test
• help wanted

CONTRIBUTORS (optional):