cmd: rework reexec detection

[mvo5]

This reworked the detection if we already re-execed or not. The logic is simply to check if the /proc/self/exe points inside the /snap/* directory. If that is the case we did already re-exec and there is no need to do anything further.

This also contains the bugfix when re-execing from snapd 2.21 which used to set SNAP_REEXEC=0.

[zyga]

This looks OK; if the tests pass then let's merge it

[codecov-io]

Codecov Report

Merging #3608 into master will decrease coverage by 0.02%.
The diff coverage is 47.05%.

@@            Coverage Diff             @@
##           master    #3608      +/-   ##
==========================================
- Coverage    74.9%   74.88%   -0.03%     
==========================================
  Files         380      380              
  Lines       32952    32961       +9     
==========================================
  Hits        24682    24682              
- Misses       6477     6485       +8     
- Partials     1793     1794       +1

Impacted Files	Coverage Δ
cmd/cmd.go	83.87% <47.05%> (-8.99%)	⬇️
overlord/ifacestate/helpers.go	62.33% <0%> (ø)	⬆️
interfaces/sorting.go	91.42% <0%> (ø)	⬆️

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