cmd,tests: fix classic confinement confusing re-execution code (#3598)

[zyga]

cmd,tests: fix classic confinement confusing re-execution code

When snapd inside the core snap is more recent than the snapd in the
classic distribution and the classic distribution supports snapd
re-execution then certain tools, namely snap and snapd will re-execute
the version of themselves found in the core snap.

To prevent endless loops as a part of this re-execution the variable
SNAP_DID_REEXEC is to 1 and tested in the re-execution mechanism.

Snaps using classic confinement do not get a mount namespace rooted at
the core (or designated base) snap and instead share the classic mount
namespace. As such their /usr/bin/snap is the one from the distribution
package.

When those two features are combined, a snap using classic confinement
will run with SNAP_DID_REEXEC=1 and /usr/bin/snap being the one from the
distribution. If such snap now runs a snap command, it will couple
potentially older /usr/bin/snap (e.g. from 2.25) with the profiles from
potentially newer /usr/bin/snapd (e.g. from 2.26.9). That specific
combination crosses the threshold where snap-seccomp begins to be used
and thus the old snap-confine cannot run new applications.

As a fix, unset SNAP_DID_REEXEC after observing the value 1, this will
ensure that the execution environment will not contain that flag and
each subsequent attempt to run /usr/bin/snap will re-consider
re-execution correctly.

Fixes: https://bugs.launchpad.net/snapd/+bug/1704860
Forum: https://forum.snapcraft.io/t/snapd-2-26-9-and-conjure-up-no-longer-work/
Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[codecov-io]

Codecov Report

❗️ No coverage uploaded for pull request base (release/2.27@7692d6a). Click here to learn what that means.
The diff coverage is 100%.

@@               Coverage Diff               @@
##             release/2.27    #3674   +/-   ##
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  Coverage                ?   74.84%           
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  Files                   ?      380           
  Lines                   ?    32931           
  Branches                ?        0           
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  Hits                    ?    24647           
  Misses                  ?     6487           
  Partials                ?     1797

Impacted Files	Coverage Δ
cmd/cmd.go	84.78% <100%> (ø)

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