graalvm: fix dead url (#44723)

pkgs/development/compilers/graalvm/default.nix

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     rec { sha1 = "396ce0c07ba2b481f25a70195c7c94922f0d1b0b"; name = "ASM_TREE5_${sha1}/asm-tree5.jar";                                                 url = mirror://maven/org/ow2/asm/asm-tree/5.0.4/asm-tree-5.0.4.jar; }
     rec { sha1 = "280c265b789e041c02e5c97815793dfc283fb1e6"; name = "LIBFFI_${sha1}/libffi.tar.gz";                                                    url = https://lafo.ssw.uni-linz.ac.at/pub/graal-external-deps/libffi-3.2.1.tar.gz; }
     rec { sha1 = "8819cea8bfe22c9c63f55465e296b3855ea41786"; name = "TruffleJSON_${sha1}/trufflejson.jar";                                             url = https://lafo.ssw.uni-linz.ac.at/pub/graal-external-deps/trufflejson-20180130.jar; }
-    rec { sha1 = "9712a8124c40298015f04a74f61b3d81a51513af"; name = "CHECKSTYLE_8.8_${sha1}/checkstyle-8.8.jar";                                       url = https://github.com/graalvm/mx/releases/download/checkstyle-8.8/checkstyle-8.8-all.jar; }
+    rec { sha1 = "9712a8124c40298015f04a74f61b3d81a51513af"; name = "CHECKSTYLE_8.8_${sha1}/checkstyle-8.8.jar";                                       url = https://lafo.ssw.uni-linz.ac.at/pub/graal-external-deps/checkstyle-8.8-all.jar; }
   ];
 
   findbugs = fetchzip {