feat(core): allow to set github details at runtime with env vars (#3708)

linuxgsm.sh

@@ -47,9 +47,9 @@ userinput2="${2}"
 ## GitHub Branch Select
 # Allows for the use of different function files
 # from a different repo and/or branch.
-githubuser="GameServerManagers"
-githubrepo="LinuxGSM"
-githubbranch="master"
+[ -n "${LGSM_GITHUBUSER}" ] && githubuser="${LGSM_GITHUBUSER}" || githubuser="GameServerManagers"
+[ -n "${LGSM_GITHUBREPO}" ] && githubrepo="${LGSM_GITHUBREPO}" || githubrepo="LinuxGSM"
+[ -n "${LGSM_GITHUBBRANCH}" ] && githubbranch="${LGSM_GITHUBBRANCH}" || githubbranch="master"
 
 # Core function that is required first.
 core_functions.sh(){