subset_data: --user-mods-dir must be globalized

### Brief summary of bug

If the user gives a local path for `--user-mods-dir`, it will be local in their `shell_commands`. This will cause a failure when they try to run the case.

### General bug information

**CTSM version you are using:** ctsm5.3.061

**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** n/a

### Important details of your setup / configuration so we can reproduce the bug

`subset_data` command, from CTSM directory:
```bash
tools/site_and_regional/subset_data region --lat1 -12 --lat2 -7 --lon1 291 --lon2 299 --create-mesh --create-domain --create-surface --create-landuse --surf-year 1850 --create-user-mods --crop --outdir subset_data_regional.crop
```

### Important output or errors that show the problem

`./check_input_data` output from case directory:
```
Loading input file list: 'Buildconf/ctsm.input_data_list'
Model ctsm missing file fsurdat = 'subset_data_regional.crop/surfdata_291.0-299.0_-12.0--7.0_hist_2000_78pfts_c250701.nc'
Model ctsm missing file flanduse_timeseries = 'subset_data_regional.crop/landuse.timeseries_291.0-299.0_-12.0--7.0_SSP2-4.5_1850-2100_78pfts_c250701.nc'
Loading input file list: 'Buildconf/cpl.input_data_list'
Model cpl missing file mesh_mask = '/glade/campaign/cesm/cesmdata/inputdata/subset_data_regional.crop/domain.lnd.fv0.9x1.25_gx1v7_291.0-299.0_-12.0--7.0_c250701_ESMF_UNSTRUCTURED_MESH.nc'
Model cpl missing file mesh_atm = '/glade/campaign/cesm/cesmdata/inputdata/subset_data_regional.crop/domain.lnd.fv0.9x1.25_gx1v7_291.0-299.0_-12.0--7.0_c250701_ESMF_UNSTRUCTURED_MESH.nc'
Model cpl missing file mesh_lnd = '/glade/campaign/cesm/cesmdata/inputdata/subset_data_regional.crop/domain.lnd.fv0.9x1.25_gx1v7_291.0-299.0_-12.0--7.0_c250701_ESMF_UNSTRUCTURED_MESH.nc'
Loading input file list: 'Buildconf/datm.input_data_list'
Model datm missing file model_meshfile = '/glade/campaign/cesm/cesmdata/inputdata/subset_data_regional.crop/domain.lnd.fv0.9x1.25_gx1v7_291.0-299.0_-12.0--7.0_c250701_ESMF_UNSTRUCTURED_MESH.nc'
Model datm missing file model_maskfile = '/glade/campaign/cesm/cesmdata/inputdata/subset_data_regional.crop/domain.lnd.fv0.9x1.25_gx1v7_291.0-299.0_-12.0--7.0_c250701_ESMF_UNSTRUCTURED_MESH.nc'
Loading input file list: 'Buildconf/mosart.input_data_list'
```