build: Add GUIX_GIT_KEYRING variable for make authenticate.

* Makefile.am (GUIX_GIT_KEYRING): New variable.
(authenticate): Use GUIX_GIT_KEYRING to select the keyring branch
reference.
* doc/contributing.texi (Building from Git): Add an example about the
use of GUIX_GIT_KEYRING.

Makefile.am

@@ -666,9 +666,11 @@ channel_intro_commit = 9edb3f66fd807b096b48283debdcddccfea34bad
 channel_intro_signer = BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA
 
 # Authenticate the current Git checkout by checking signatures on every commit.
+GUIX_GIT_KEYRING = origin/keyring
 authenticate:
 	$(AM_V_at)echo "Authenticating Git checkout..." ;	\
 	guix git authenticate					\
+	    --keyring=$(GUIX_GIT_KEYRING)			\
 	    --cache-key=channels/guix --stats			\
 	    "$(channel_intro_commit)" "$(channel_intro_signer)"
 

doc/contributing.texi

@@ -140,6 +140,16 @@ make authenticate
 
 The first run takes a couple of minutes, but subsequent runs are faster.
 
+Or, when your configuration for your local Git repository doesn't match
+the default one, you can provide the reference for the @code{keyring}
+branch through the variable @code{GUIX_GIT_KEYRING}.  The following
+example assumes that you have a Git remote called @samp{myremote}
+pointing to the official repository:
+
+@example
+make authenticate GUIX_GIT_KEYRING=myremote/keyring
+@end example
+
 @quotation Note
 You are advised to run @command{make authenticate} after every
 @command{git pull} invocation.  This ensures you keep receiving valid