cmd/snap-bootstrap: mount snapd snap directly to /snap

[alfonsosanchezbeato]

On first boot we had to mount the snapd snap to /run/mnt/base. Avoid this duplicate mount by mounting directly to /snapd/snapd/, which will allow also some simplifications in the base script that runs on first boot.

[alfonsosanchezbeato]

These changes depend on canonical/core-base#255 , which needs to land first. Furthermore, we would need the base change in core20+ and have all published revisions in stable before this can be released. With this we would make sure that when you create new seeds we would also have the needed base changes so the snap-bootstrap change would not break things.

[codecov]

Codecov Report

Attention: Patch coverage is 67.56757% with 24 lines in your changes missing coverage. Please review.

Project coverage is 78.87%. Comparing base (ac897ee) to head (60df4c0).
Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/snap-bootstrap/cmd_initramfs_mounts.go	67.39%	9 Missing and 6 partials ⚠️
cmd/snap-bootstrap/initramfs_systemd_mount.go	67.85%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14548      +/-   ##
==========================================
+ Coverage   78.85%   78.87%   +0.01%     
==========================================
  Files        1079     1080       +1     
  Lines      145615   145903     +288     
==========================================
+ Hits       114828   115074     +246     
- Misses      23601    23630      +29     
- Partials     7186     7199      +13     

Flag	Coverage Δ
unittests	78.87% <67.56%> (+0.01%)	⬆️

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