fix(goenv.GOMOD): never rely on the environment variable

[RomainMuller]

The value of go env GOMOD changes depending on the working directory
where the command is executed. Instead of relying on the environment
variable, always spawn go env GOMOD in an explicitly selected
directory.

[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 60.86%. Comparing base (3d60a25) to head (7a7eade).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
instrument/opname.go	88.88%	1 Missing and 1 partial ⚠️
internal/ensure/requiredversion.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
+ Coverage   60.85%   60.86%   +0.01%     
==========================================
  Files         174      175       +1     
  Lines       12341    12343       +2     
==========================================
+ Hits         7510     7513       +3     
+ Misses       4350     4349       -1     
  Partials      481      481              

Components	Coverage Δ
Generators	76.91% <ø> (ø)
Instruments	88.05% <88.88%> (ø)
Go Driver	73.94% <100.00%> (+0.47%)	⬆️
Toolexec Driver	70.66% <ø> (ø)
Aspects	71.85% <ø> (ø)
Injector	73.74% <ø> (ø)
Job Server	64.02% <ø> (ø)
Integration Test Suite	54.24% <ø> (ø)
Other	60.86% <88.46%> (+0.01%)	⬆️

Files with missing lines	Coverage Δ
instrument/instrument.go	66.66% <ø> (-11.12%)	⬇️
internal/goenv/goenv.go	76.92% <100.00%> (+10.25%)	⬆️
internal/pin/pin.go	57.86% <100.00%> (ø)
internal/ensure/requiredversion.go	70.00% <80.00%> (+0.23%)	⬆️
instrument/opname.go	88.88% <88.88%> (ø)