chore(build): improve error wording if no package repo could be detected

Not being in any official repo does not necessarily mean this is a new
package. One could simply be building an AUR or custom local package.
Make the message less confusing in such case.

src/lib/build/build.sh

@@ -298,7 +298,7 @@ pkgctl_build() {
 				die 'failed to get pacman repo'
 			fi
 			if [[ -z "${pkgrepo}" ]]; then
-				die 'unknown repo, please specify --repo for new packages'
+				die 'unknown repo, specify --repo for packages not currently in any official repo'
 			fi
 		fi