snap: Incorporate v3.0.6 selective-checkout scriptlet for ease of sta…

…ble 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

Signed-off-by: 林博仁(Buo-ren Lin) <[email protected]>