many: do not use nss when looking up for users/groups from snapd snap

[valentindavid]

~~Depends on #13370~~

When snapd runs as a snap, it has its own runtime. This may not have NSS plugins needed for the host. For example to get users from AD/LDAP/Kerberos, or systemd-homed, or custom user databses. In general we can use tag osusergo to make go not to use the local configuration (i.e. /etc/nsswitch.conf), however, even if it is fine for most databases, we really need users and groups to be resolved with the host configuration.

To be able to load correctly plugins, we expect the host system to provide getent. And we query passwd and group databases through this command.

In the future we should connect the systemd-userdb if it is running and use getent only as fallback.

[valentindavid]

This branch depends on #13517 to be properly tested.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.74%. Comparing base (ea13a33) to head (b9df3a3). Report is 37 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13776      +/-   ##
==========================================
+ Coverage   78.73%   78.74%   +0.01%     
==========================================
  Files        1055     1061       +6     
  Lines      138275   139197     +922     
==========================================
+ Hits       108866   109609     +743     
- Misses      22588    22736     +148     
- Partials     6821     6852      +31     

Flag	Coverage Δ
unittests	78.74% <ø> (+0.01%)	:arrow_up:

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