gadget/install: switch mount flags for ubuntu-save to no{dev,exec,suid}

[Meulengracht]

This makes us mount ubuntu-save with no{dev,exec,suid} during install-mode as well

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.03%. Comparing base (96ea7b0) to head (0c378f3).
Report is 99 commits behind head on master.

Files with missing lines	Patch %	Lines
gadget/install/install.go	72.72%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14740      +/-   ##
==========================================
+ Coverage   78.95%   79.03%   +0.08%     
==========================================
  Files        1084     1087       +3     
  Lines      146638   147708    +1070     
==========================================
+ Hits       115773   116747     +974     
- Misses      23667    23732      +65     
- Partials     7198     7229      +31     

Flag	Coverage Δ
unittests	79.03% <72.72%> (+0.08%)	⬆️

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[zyga]

LGTM with one suggestion

[bboozzoo]

LGTM

[alfonsosanchezbeato]

LGTM - I was wondering if somebody would do something crazy to ubuntu-save from the install-device hook, but not sure if that makes sense.