Check for MRAN differently

tests/r/mran/verify

@@ -4,6 +4,6 @@ library('ggplot2')
 print(version)
 
 # Fail if MRAN isn't the configured CRAN mirror
-if (!grepl("https://mran.microsoft.com", options()$CRAN, fixed=TRUE)) {
+if (!(startsWith(options()$repos["CRAN"], "https://mran.microsoft.com"))) {
   quit("yes", 1)
 }

tests/r/r4.0/verify

@@ -8,6 +8,6 @@ if (!(version$major == "4" && as.double(version$minor) >= 0 && as.double(version
 }
 
 # The date we have chosen should give us an MRAN mirror
-if (!grepl("https://mran.microsoft.com", options()$CRAN, fixed=TRUE)) {
+if (!(startsWith(options()$repos["CRAN"], "https://mran.microsoft.com"))) {
   quit("yes", 1)
-}
+}