build: use tool from configure

This way we make sure the tool will be the one we actually configured
before going through sudo.
diff --git a/configure.ac b/configure.ac
index ca94bf8..932e87d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -31,6 +31,7 @@
 AC_PROG_LN_S
 PKG_PROG_PKG_CONFIG
 AC_PATH_PROG([XSLTPROC], [xsltproc])
+AC_PATH_PROG([MKOSI], [mkosi])
 
 AC_PROG_CC_C99