steam_support: add unshare CLONE_NEWNS to steam_support interface

[ashuntu]

As per the discussion in the Snapcraft forum, this PR adds unshare CLONE_NEWNS to the steam_support builtin interface.

This change is essentially exactly @alexmurray's suggestion.

[codecov-commenter]

Codecov Report

Merging #13038 (23e7372) into master (4cab726) will decrease coverage by 0.01%.
Report is 35 commits behind head on master.
The diff coverage is 78.72%.

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@@            Coverage Diff             @@
##           master   #13038      +/-   ##
==========================================
- Coverage   78.69%   78.68%   -0.01%     
==========================================
  Files         999     1001       +2     
  Lines      123909   124304     +395     
==========================================
+ Hits        97511    97812     +301     
- Misses      20265    20337      +72     
- Partials     6133     6155      +22     

Flag	Coverage Δ
unittests	78.68% <78.72%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
cmd/snap/cmd_pack.go	86.66% <ø> (ø)
interfaces/builtin/kvm.go	93.02% <0.00%> (ø)
interfaces/builtin/steam_support.go	100.00% <ø> (ø)
interfaces/seccomp/backend.go	83.26% <0.00%> (+1.04%)	⬆️
overlord/assertstate/assertstate.go	76.76% <0.00%> (ø)
overlord/configstate/configcore/netplan.go	9.37% <0.00%> (-0.15%)	⬇️
overlord/snapstate/flags.go	100.00% <ø> (ø)
usersession/agent/rest_api.go	0.00% <0.00%> (ø)
overlord/snapstate/storehelpers.go	77.00% <62.50%> (-0.87%)	⬇️
osutil/kcmdline/kcmdline.go	88.61% <64.06%> (ø)
... and 31 more

... and 4 files with indirect coverage changes

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[alexmurray]

LGTM.

[mvo5]

Thank you

[kenvandine]

This looks good to me and I have verified it fixes the issue reported on the steam snap.