CI: Pin cc in nightly job

A recent commit (rust-lang/cc-rs#780) broke our CI nightly job. Pin to a
version before that was merged.

This should be temporary until the issue is fixed.

Author: tcharding

contrib/test.sh

@@ -11,6 +11,8 @@ rustc --version
 NIGHTLY=false
 if cargo --version | grep nightly; then
     NIGHTLY=true
+    # Pin dependencies as required if we are using nightly toolchain.
+    cargo update -p cc --precise 1.0.79
 fi
 
 # Pin dependencies as required if we are using MSRV toolchain.