cmd/snap-update-ns: fix mount rules for font sharing (2.29)

[zyga]

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

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

[codecov-io]

Codecov Report

Merging #4137 into release/2.29 will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           release/2.29   #4137   +/-   ##
============================================
  Coverage          75.8%   75.8%           
============================================
  Files               433     433           
  Lines             37250   37250           
============================================
  Hits              28236   28236           
  Misses             7042    7042           
  Partials           1972    1972

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