cmd/snap-update-ns: misc cleanups

[zyga]

This branch cleans up a few things I found while working on other parts of the code

[chipaca]

you can write this like

var changePerform = (*Change).Perform

[zyga]

I did this in a follow-up

[codecov-io]

Codecov Report

Merging #4223 into master will increase coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4223      +/-   ##
==========================================
+ Coverage   75.86%   75.98%   +0.11%     
==========================================
  Files         440      440              
  Lines       38247    38377     +130     
==========================================
+ Hits        29016    29159     +143     
+ Misses       7218     7207      -11     
+ Partials     2013     2011       -2

Impacted Files	Coverage Δ
cmd/snap-update-ns/main.go	39.06% <ø> (+0.6%)	⬆️
cmd/snap-update-ns/change.go	95.29% <0%> (-2.3%)	⬇️
snap/info_snap_yaml.go	97.03% <0%> (+2.81%)	⬆️
snap/info.go	86.05% <0%> (+5.3%)	⬆️

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