cmd/snap-confine: don't share /etc/nsswitch from host

[zyga]

This patch corrects name resolution errors on Arch where host's
/etc/nsswitch.conf contains references to modules not present in the
core snap. This is again one of the bugs originating from sharing all of
/etc blindly and then undoing the damage but for now it is easier to add
an exception than to undo this.

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

[codecov-io]

Codecov Report

Merging #3650 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #3650      +/-   ##
=========================================
+ Coverage    75.2%   75.2%   +<.01%     
=========================================
  Files         387     387              
  Lines       33452   33452              
=========================================
+ Hits        25157   25159       +2     
+ Misses       6482    6481       -1     
+ Partials     1813    1812       -1

Impacted Files	Coverage Δ
cmd/snap/cmd_aliases.go	93.33% <0%> (-1.67%)	⬇️
overlord/snapstate/snapstate.go	80.46% <0%> (+0.26%)	⬆️

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[jdstrand]

+1