gnu: gdb: Distinguish between the "pinned" and the "default" version.

* gnu/packages/gdb.scm (gdb/pinned): New variable.
* gnu/packages/rust.scm (rust)[native-inputs]: Use it instead of GDB.

gnu/packages/gdb.scm

@@ -161,11 +161,15 @@ written in C, C++, Ada, Objective-C, Pascal and more.")
     ;; GDB 12 builds fine on GNU/Hurd.
     (supported-systems %supported-systems)))
 
-(define-public gdb
+(define-public gdb/pinned
   ;; This is the fixed version that packages depend on.  Update it rarely
   ;; enough to avoid massive rebuilds.
   gdb-11)
 
+(define-public gdb
+  ;; The "default" version.
+  gdb-11)
+
 (define-public gdb-minimal
   (package/inherit gdb-12
     (name "gdb-minimal")

gnu/packages/rust.scm

@@ -788,7 +788,7 @@ safety and thread safety guarantees.")
                     (format #f "prefix = ~s" (assoc-ref outputs "rustfmt"))))
                  (invoke "./x.py" "install" "rustfmt")))))))
       ;; Add test inputs.
-      (native-inputs (cons* `("gdb" ,gdb)
+      (native-inputs (cons* `("gdb" ,gdb/pinned)
                             `("procps" ,procps)
                             (package-native-inputs base-rust))))))