Stable backports (#8055)

* CI: Fix cargo cache (#7968)

* Fix cache

Blocking waiting for file lock on the registry index

* Only clean locked cargo cache on windows

* fixed ethstore sign (#8026)

* fix cache & snapcraft CI build (#8052)

after successful testing it is necessary to port in a ```beta``` and ```stable```

*  Add MCIP-6 Byzyantium transition to Musicoin spec (#7841)

* Add test chain spec for musicoin byzantium testnet

* Add MCIP-6 Byzyantium transition to Musicoin spec

* Update mcip6_byz.json

* ethcore: update musicoin byzantium block number

* ethcore: update musicoin byzantium block number

* ethcore: update musicoin bootnodes

* Update musicoin.json

* Update musicoin.json

* More bootnodes.