ci: Remove ccache requirement on the host

ccache is only needed to create the cache dir on the host, if it didn't
already exist. The same can be achieved with mkdir, so just use that
instead.

ci/test/04_install.sh

@@ -33,7 +33,7 @@ if [ "$TRAVIS_OS_NAME" == "osx" ]; then
 fi
 
 mkdir -p "${BASE_SCRATCH_DIR}"
-ccache echo "Creating ccache dir if it didn't already exist"
+mkdir -p "${CCACHE_DIR}"
 
 if [ ! -d ${DIR_QA_ASSETS} ]; then
   git clone https://github.com/bitcoin-core/qa-assets ${DIR_QA_ASSETS}