Make spawn-rpc "return" url instead of logging/printing it

[beruda]

Current implementation just prints the URL instead of returning it, so the current command can't be used in bash scripts.

[beruda]

Running

test=$(tenderly version)

won't print anything, but echo $test after it will.

But running

rpc=$(tenderly devnet spawn-rpc --project testerly --account beruda --access_key $API_KEY --template all-purpose-support-testing)

will print the URL, but it won't be written to the rpc env variable; i.e. echo $rpc won't print the URL.

So I just changed this command to return the value using fmt.Printf just like the version command.

[daleksov]

Additional flag is way more safe. So I'm OK with this approach, since we are not going to affect any of the existing integrations.