gnu: gcc-toolchain: Remove duplicate gcc-toolchain@10.

This is a followup to 4796b5d6d1.

Fixes the "fold-available-packages with/without cache" test in
'tests/packages.scm'.

* gnu/packages/commencement.scm (gcc-toolchain-8): Explicitly refer to
GCC-8.
(gcc-toolchain-10): Turn into an alias for GCC-TOOLCHAIN.

gnu/packages/commencement.scm

@@ -3808,13 +3808,13 @@ is the GNU Compiler Collection.")
   (make-gcc-toolchain gcc-7))
 
 (define-public gcc-toolchain-8
-  gcc-toolchain)
+  (make-gcc-toolchain gcc-8))
 
 (define-public gcc-toolchain-9
   (make-gcc-toolchain gcc-9))
 
 (define-public gcc-toolchain-10
-  (make-gcc-toolchain gcc-10))
+  gcc-toolchain)
 
 (define-public gcc-toolchain-11
   (make-gcc-toolchain gcc-11))