No bug - build cbindgen for macos using rust 1.32. r=froydnj

1.32 includes rust-lang/rust#49219, which means new
cbindgen no longer depends on compiler internals, which fixes some reported
build issues on IRC.

UltraBlame original commit: 8e48fdd65475c48fdc2f6af9e359c3d07bee66e5

Author: marco-c

taskcluster/ci/toolchain/cbindgen.yml

@@ -39,7 +39,7 @@ macosx64-cbindgen:
         - linux64-cctools-port
         - linux64-clang-7
         - linux64-llvm-dsymutil
-        - linux64-rust-macos-1.31
+        - linux64-rust-macos-1.32
 
 win64-cbindgen:
     treeherder: