cmd/snap: run inhibition flow in goroutine

[ZeyadYasser]

This avoids the deadlock scenario:

• snap run holds inhibition lock.
• snapd holds state lock (link-snap).
• snapd tries to hold inhibition lock but waits for snap run to release lock.
• snap run calls the REST API (i.e. for notices, connections) which tries to hold state lock.
• snap run is now waiting for snapd to release state lock (to send the notice) and snapd is waiting for snap run to release the inibition lock.
• we have a deadlock and snapd state is locked.

Running inhibition flow in a goroutine allows snap run to release the inhibition lock.

Fixes: https://bugs.launchpad.net/snapd/+bug/2084730

[Meulengracht]

@bboozzoo @pedronis @ZeyadYasser should we close this PR and focus on the state unlocker PR?

[ernestl]

Blocked by decision.

[ernestl]

This is not important for 2.67, We decided to remove this from 2.67 and rather release as .x or 2.68.