snap: Incorporate v3.0.6 selective-checkout scriptlet for ease of stable snap publishing

This scriptlet allows stable builds to be always available for the
downstream snap publisher to promote to the `stable` channel on
the Snap Store.

Higher risk builds(e.g. beta/edge) will only occur when the lower risk
builds(rc/stable) are promoted.

Refer the following Snapcraft Forum topic for more info:

    Selective-checkout: Check out the tagged release revision if it isn't promoted to the stable channel - doc - snapcraft.io
    https://forum.snapcraft.io/t/selective-checkout-check-out-the-tagged-release-revision-if-it-isnt-promoted-to-the-stable-channel/10617

Here's a snippet of the scriptlet in action:

```
:: selective-checkout: INFO: Detecting stable releases...
:: selective-checkout: determine_stable_release_details: DEBUG: Last stable release tag determines to be "5.4.1".
:: selective-checkout: determine_stable_release_details: DEBUG: Last stable version determines to be "5.4.1".
:: selective-checkout: snap_query_version: DEBUG: Checking what snap revisions are available for the tesseract snap at the stable release channel...
:: selective-checkout: determine_stable_release_details: DEBUG: Last stable version on the snap store determines to be "".
:: selective-checkout: INFO: Detecting release candidate releases...
:: selective-checkout: DEBUG: Last release candidate release tag determines to be "5.4.0-rc2".
:: selective-checkout: DEBUG: Last release candidate version determines to be "5.4.0-rc2".
:: selective-checkout: snap_query_version: DEBUG: Checking what snap revisions are available for the tesseract snap at the candidate release channel...
:: selective-checkout: DEBUG: Last release candidate version on the snap store determines to be "".
:: selective-checkout: INFO: Detecting beta releases...
:: selective-checkout: DEBUG: Last beta release tag determines to be "5.0.0-beta-20210916".
:: selective-checkout: DEBUG: Last beta version determines to be "5.0.0-beta-20210916".
:: selective-checkout: snap_query_version: DEBUG: Checking what snap revisions are available for the tesseract snap at the beta release channel...
:: selective-checkout: DEBUG: Last beta version on the snap store determines to be "".
:: selective-checkout: determine_which_version_to_build: DEBUG: Stable version(5.4.1) is newer than the one on the Snap Store(none), stable release will be built.
:: selective-checkout: Info: The last tagged stable release(5.4.1) hasn't been promoted to the stable channel() on the Snap Store yet, checking out 5.4.1.
```

Signed-off-by: 林博仁(Buo-ren Lin) <buo.ren.lin@gmail.com>

Author: brlin-tw