check_up_to_date() also checks for subworkflows

[mirpedrol]

The check_up_to_date() function from modules_json.py also checks for subworkflows.
This also fixes the issue when installing a subworkflow from a new modules repo. To test:

nf-core create # Create a new test pipeline
nf-core subworkflows --git-remote https://gitlab.com/nf-core/modules-test.git --branch subworkflows install bam_sort_samtools # try installing a subworkflow from a new remote

close #1917

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1934 (a4b4645) into dev (865cae8) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev    #1934   +/-   ##
=======================================
  Coverage   60.03%   60.04%           
=======================================
  Files          37       37           
  Lines        4642     4640    -2     
=======================================
- Hits         2787     2786    -1     
+ Misses       1855     1854    -1     

Impacted Files	Coverage Δ
nf_core/subworkflows/install.py	14.50% <0.00%> (+0.21%)	⬆️
nf_core/modules/test_yml_builder.py	49.09% <0.00%> (-0.46%)	⬇️

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