interfaces/time*_control: explicitly deny noisy read on /proc/1/environ

[jdstrand]

systemd utilities look at /proc/1/environ to see if running in a container, but
they will fallback gracefully. No other interfaces allow this denial, so no
problems with silencing it for now. Note that allowing this triggers a 'ptrace
trace peer=unconfined' denial, which we want to avoid.

Reference:
https://forum.snapcraft.io/t/managing-time-date-and-timezone-in-ubuntu-core/408/44

[zyga]

+1

[codecov-io]

Codecov Report

Merging #4216 into master will increase coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4216      +/-   ##
=========================================
+ Coverage   75.86%   76.1%   +0.23%     
=========================================
  Files         439     440       +1     
  Lines       38232   38532     +300     
=========================================
+ Hits        29004   29323     +319     
+ Misses       7216    7179      -37     
- Partials     2012    2030      +18

Impacted Files	Coverage Δ
interfaces/builtin/time_control.go	100% <ø> (ø)	⬆️
interfaces/builtin/timeserver_control.go	100% <ø> (ø)	⬆️
interfaces/builtin/timezone_control.go	100% <ø> (ø)	⬆️
interfaces/builtin/browser_support.go	76.08% <0%> (-0.23%)	⬇️
interfaces/builtin/opengl.go	100% <0%> (ø)	⬆️
overlord/snapstate/readme.go	75% <0%> (ø)
overlord/snapstate/snapmgr.go	87.83% <0%> (+3.82%)	⬆️
store/store.go	83.89% <0%> (+3.89%)	⬆️

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