ci: Set CCACHE_SLOPPINESS as required for using PCH

While the solution of setting CCACHE_SLOPPINESS in the makefiles seems
to work locally, the CI has been reporting high miss rates since the
PCH patch landed. Possibly the CI environment has CCACHE_SLOPPINESS set
to the empty string, which prevents Make ?= to apply.

.github/workflows/build.yml

@@ -17,6 +17,8 @@ env:
   CCACHE_COMPRESS: 1
   CCACHE_DIR: ${{ github.workspace }}/.ccache
   CCACHE_LIMIT_MULTIPLE: 0.95
+  CCACHE_SLOPPINESS: pch_defines,time_macros
+  CCACHE_LOGFILE: ccache.log
 
 defaults:
   run:

ci/ci-script.bash

@@ -48,8 +48,10 @@ if [ "$CI_BUILD_STAGE_NAME" = "build" ]; then
     autoconf
     ./configure --enable-longtests --enable-ccwarn ${CI_M32:+--enable-m32}
     ccache -z
+    ccache -p
     "$MAKE" -j "$NPROC" -k
     ccache -s
+    cat ccache.log
     if [ "$CI_OS_NAME" = "osx" ]; then
       file bin/verilator_bin
       file bin/verilator_bin_dbg