Correct spack-stack path for Gaea-C6

[DavidBurrows-NCO]

Updates the spack-stack path for Gaea-C6 to follow ufs-wx-model.
Resolves #1082
Refs NOAA-EMC/global-workflow#3011

[WenMeng-NOAA]

@DavidBurrows-NCO Have you tested on building UPP executable on Gaea-C6 given that Gaea is not included in the UPP RTs?

[DavidBurrows-NCO]

@WenMeng-NOAA That's correct; I retested after correcting the spack-stack path...here's a snippet from that:

./compile_upp.sh 
Running "module reset". Resetting modules to system default. The following $MODULEPATH directories have been removed: /ncrc/proj/epic/rocoto/modulefiles
0x00007f3869eeabe6 in wait4 () from /lib64/libc.so.6
$1 = 0
$2 = -1
[Inferior 1 (process 2366730) detached]
Building for machine gaeac6, compiler intel
...
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/include/gtg_ctlblk.mod
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/include/icingpotential.mod
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/include/gtg_config.mod
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/include/native_endianness.mod
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/lib/libupp.a
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/lib/cmake/upp/upp-config.cmake
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/lib/cmake/upp/upp-config-version.cmake
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/lib/cmake/upp/upp-targets.cmake
-- Installing: /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/tests/install/lib/cmake/upp/upp-targets-release.cmake
+ rm -rf /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/exec
+ mkdir -p /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/exec
+ cp ../install/bin/upp.x /gpfs/f6/bil-fire8/scratch/David.Burrows/double_check_upp_build/UPP/exec/.
+ [[ NO == YES ]]

[WenMeng-NOAA]

No need for the UPP RTs. This PR is ready for merging.