Merge #678: CI: Pin cc for MSRV build

fb0fc1d CI: Pin cc for MSRV build (Tobin C. Harding)

Pull request description:

  The `cc` dependency just stopped working for MSRV build with error

      error: package `cc v1.0.96` cannot be built because it requires
      rustc 1.63 or newer, while the currently active rustc version is
      1.56.1

  Pin `cc` back to an earlier version for MSRV build.

ACKs for top commit:
  apoelstra:
    ACK fb0fc1d

Tree-SHA512: 95b952ed2c5c3ac0580fbc1017b6fb8b50c6ced1b5b80b58749fa734b5256d03e82a9baa2305dc37a642d9ae732a60a0b68d783c88f54b480a91b1e1eaa86fe7

contrib/test.sh

@@ -9,8 +9,15 @@ rustc --version
 
 # Cache the toolchain we are using.
 NIGHTLY=false
+MSRV=false
 if cargo --version | grep nightly; then
     NIGHTLY=true
+elif cargo --version | grep "1\.56"; then
+    MSRV=true
+fi
+
+if [ "$MSRV" = true ]; then
+    cargo update -p cc --precise 1.0.79
 fi
 
 # Format if told to