Don't use 'git -C' since apparently older versions of git don't have it

<nelgin> git -C /home/bbs/sbbs/repo remote set-url origin https://gitlab.synchro.net/main/sbbs.git
<nelgin> Unknown option: -C
<nelgin> using GNUMakefile on CentOS 7

This partially reverts commit 34909899

install/GNUmakefile

@@ -247,9 +247,10 @@ $(REPODIR):
 	git clone https://gitlab.com/SynchronetBBS/sbbs.git $(REPODIR) \
 		|| git clone https://github.com/SynchronetBBS/sbbs.git $(REPODIR) \
 		|| git clone https://gitlab.synchro.net/main/sbbs.git $(REPODIR)
-	git -C $(REPODIR) remote set-url origin https://gitlab.synchro.net/main/sbbs.git
+	cd $(REPODIR)
+	git remote set-url origin https://gitlab.synchro.net/main/sbbs.git
 ifdef TAG
-	git -C $(REPODIR) checkout tags/$(TAG)
+	git checkout tags/$(TAG)
 endif
 
 src: $(REPODIR)