mom5: use a more accurate name for an environment variable

[harshula]

While debugging an issue, I found that the variable name $SPACK_BUILD confusing because we use Spack to build MOM5 regardless of whether that variable is set to true of false.

[harshula]

Before merging this to main, I have to commit the following to MOM5's development branch:

diff --git a/exp/MOM_compile.csh b/exp/MOM_compile.csh
index b095b22..272054a 100755
--- a/exp/MOM_compile.csh
+++ b/exp/MOM_compile.csh
@@ -156,7 +156,7 @@ if ( $cosima_version ) then
     set cppDefs = "$cppDefs -DCOSIMA_VERSION"
 endif
 
-if ( ! $?SPACK_BUILD ) then
+if ( ! $?SPACK_FMS_EXTERNAL ) then
     echo "Building type=$type with internal FMS"
     source ./FMS_compile.csh
     set includes = "-I$code_dir/shared/include -I$executable:h:h/lib_FMS -I$executable:h:h/lib_ocean"
@@ -264,7 +264,7 @@ if( $type == ACCESS-OM || $type == ACCESS-ESM) then
     set srcList = ( $srcList access/shared )
 endif
 
-if ( ! $?SPACK_BUILD ) then
+if ( ! $?SPACK_FMS_EXTERNAL ) then
     set libs = "$libs $executable:h:h/lib_FMS/lib_FMS.a"
     if( $type == ACCESS-CM || $type == ACCESS-ESM || $type == ACCESS-OM || $type == ACCESS-OM-BGC) then
         set includes = "$includes -I$executable:h:h/lib_FMS"

diff --git a/exp/ocean_compile.csh b/exp/ocean_compile.csh
index fc17630..fda7bcf 100644
--- a/exp/ocean_compile.csh
+++ b/exp/ocean_compile.csh
@@ -10,7 +10,7 @@ if( $type == ACCESS-OM || $type == ACCESS-CM || $type == ACCESS-OM-BGC || $type
         set srcList = ( $srcList mom5/ocean_csiro_bgc )
     else if ( $type ==  ACCESS-OM || $type == ACCESS-ESM ) then
 
-        if ( ! $?SPACK_BUILD ) then
+        if ( ! $?SPACK_FMS_EXTERNAL ) then
             set srcList = ( $srcList mom5/ocean_bgc access/generic_tracers/generic_tracers access/generic_tracers/mocsy/src )
         else
             set srcList = ( $srcList mom5/ocean_bgc )

[aidanheerdegen]

LGTM.