feat: add shfmt step to GitHub Actions workflow for code formatting

[shunk031]

Once shfmt supports zsh (ref. mvdan/sh#120), I will address this PR.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.40%. Comparing base (ef9ff45) to head (73bbd59).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
install/common/tpm.sh	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
+ Coverage   73.98%   74.40%   +0.41%     
==========================================
  Files           6        6              
  Lines         123      125       +2     
==========================================
+ Hits           91       93       +2     
  Misses         32       32              

Flag	Coverage Δ
ubuntu-latest-client	74.40% <80.00%> (+0.41%)	⬆️
ubuntu-latest-server	74.40% <80.00%> (+0.41%)	⬆️

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