fix(cmd/gno/clean): allow to run gno clean -modcache from anywhere + rename and use gnomod.ModCachePath + tmp GNOHOME in main tests

[n0izn0iz]

• In gno clean, run the -modcache case before checking for presence of a gno.mod to allow to run gno clean -modcache from anywhere (like go)
• Refactor gno clean -modcache to use the gnomod.GetGnoModPath helper to get the modcache path
• Rename gnomod.GetGnoModPath -> gnomod.ModCachePath
• Improve gno cmd tests by using a tmp GNOHOME instead of the system one

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 63.35%. Comparing base (d73b6c6) to head (b55bac4).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
gnovm/cmd/gno/clean.go	72.72%	2 Missing and 1 partial ⚠️
gnovm/pkg/gnomod/gnomod.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3083      +/-   ##
==========================================
+ Coverage   63.33%   63.35%   +0.01%     
==========================================
  Files         548      548              
  Lines       78680    78681       +1     
==========================================
+ Hits        49835    49846      +11     
+ Misses      25481    25473       -8     
+ Partials     3364     3362       -2     

Flag	Coverage Δ
contribs/gnodev	61.16% <ø> (ø)
contribs/gnofaucet	14.82% <ø> (ø)
gno.land	67.13% <ø> (ø)
gnovm	67.92% <73.33%> (+0.02%)	⬆️
misc/genstd	79.72% <ø> (ø)
tm2	62.45% <ø> (+0.02%)	⬆️

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