Add snap-installed bitcoind compatibility to startup_regtest.sh

[endothermicdev]

trafficstars

Would like to make regtest demos a little easier.

[cdecker]

Shellcheck isn't happy:

In contrib/startup_regtest.sh line 162:
	alias bt-cli="$BITCOINCLI -regtest"
                      ^---------^ SC2139: This expands when defined, not when used. Consider escaping.

For more information:
  https://www.shellcheck.net/wiki/SC2139 -- This expands when defined, not wh...
make: *** [Makefile:553: check-shellcheck] Error 123

[cdecker]

ACK https://github.com/ElementsProject/lightning/pull/5653/commits/38334000b96e2a66b7916fa78789ab78f377daaf

[endothermicdev]

I added a shell-check ignore statement to silence the lint warning, however when testing again with the latest snap-installed bitcoin-core, the standard naming conventions (bitcoind, bitcoin-cli) were used along with the .bitcoin directory. This PR is now unnecessary as the regtest_startup script once again works with the standard installation instructions.