cmd: fix tests that assume /snap mount

[zyga]

This patch fixes tests on Arch where snaps are mounted on
/var/lib/snapd/snap and tests are not mocking that aspect. The fix is
universal and just switches to the variable directly.

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[codecov-io]

Codecov Report

Merging #3644 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3644      +/-   ##
==========================================
+ Coverage    75.2%   75.21%   +<.01%     
==========================================
  Files         387      387              
  Lines       33452    33452              
==========================================
+ Hits        25159    25160       +1     
+ Misses       6481     6480       -1     
  Partials     1812     1812

Impacted Files	Coverage Δ
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️

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