ci: add one more pin

We should really move to a system of lockfiles (and more generally move
to the rust-bitcoin-maintainer-tools set of scripts since the existing
scripts are badly broken -- a "fuzz" job that sets an unused DO_LINT
variable and never fuzzes; a  DO_FMT flag in contrib/test.sh which is
never set, etc) but this is an easy fix for now.

Author: apoelstra

contrib/test.sh

@@ -11,6 +11,7 @@ rustc --version
 if cargo --version | grep "1\.58"; then
     cargo update -p byteorder --precise 1.4.3
     cargo update -p cc --precise 1.0.94
+    cargo update -p ppv-lite86 --precise 0.2.17
 fi
 
 # Format if told to