cmd/snap-update-ns: fix mount rules for font sharing

[zyga]

The mount rules should refer to directories, not files (the traling
slash is relevant).

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

[jdstrand]

LGTM, thanks!

[jdstrand]

s/able to/able to access/

[zyga]

This is coming from a patch by @sergiocazzolato

[codecov-io]

Codecov Report

Merging #4136 into master will increase coverage by 0.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #4136      +/-   ##
==========================================
+ Coverage   75.62%   75.63%   +0.01%     
==========================================
  Files         435      435              
  Lines       37583    37613      +30     
==========================================
+ Hits        28421    28450      +29     
+ Misses       7173     7170       -3     
- Partials     1989     1993       +4

Impacted Files	Coverage Δ
cmd/snap-update-ns/main.go	0% <0%> (ø)	⬆️
cmd/snap-update-ns/change.go	97.46% <100%> (ø)	⬆️
interfaces/builtin/serial_port.go	65.51% <0%> (-1.48%)	⬇️
overlord/ifacestate/helpers.go	60.26% <0%> (+0.66%)	⬆️
wrappers/desktop.go	78.94% <0%> (+6.78%)	⬆️

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