guix build: Fix relative file name canonicalization for '--root'.

Fixes <https://bugs.gnu.org/35271>.
Reported by rendaw <7e9wc56emjakcm@s.rendaw.me>.

* guix/scripts/build.scm (register-root): When ROOT is a relative file
name, append the basename of ROOT, not ROOT itself.
* tests/guix-build.sh: Add test.

guix/scripts/build.scm

@@ -119,7 +119,7 @@ found.  Return #f if no build log was found."
   (let* ((root (if (string-prefix? "/" root)
                    root
                    (string-append (canonicalize-path (dirname root))
-                                  "/" root))))
+                                  "/" (basename root)))))
     (catch 'system-error
       (lambda ()
         (match paths

tests/guix-build.sh

@@ -183,6 +183,13 @@ then false; else true; fi
 
 rm -f "$result"
 
+# Check relative file name canonicalization: <https://bugs.gnu.org/35271>.
+mkdir "$result"
+guix build -r "$result/x" -e '(@@ (gnu packages bootstrap) %bootstrap-guile)'
+test -x "$result/x/bin/guile"
+rm "$result/x"
+rmdir "$result"
+
 # Cross building.
 guix build coreutils --target=mips64el-linux-gnu --dry-run --no-substitutes