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.

.cirrus.yml

@@ -45,8 +45,6 @@ task:
     folder: "/tmp/ccache_dir"
   depends_built_cache:
     folder: "/tmp/cirrus-ci-build/depends/built"
-  depends_sdk_cache:
-    folder: "/tmp/cirrus-ci-build/depends/sdk-sources"
   install_script:
     - apt-get update
     - apt-get -y install git bash ccache

ci/README.md

@@ -12,7 +12,7 @@ To allow for a wide range of tested environments, but also ensure reproducibilit
 requires `docker` to be installed. To install all requirements on Ubuntu, run
 
 ```
-sudo apt install docker.io ccache bash git
+sudo apt install docker.io bash git
 ```
 
 To run the default test stage,

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}