Revert "Disable ccache on arm64 build"

This reverts commit e8124729fb.

It worked once, but soon after that it stopped working again
https://travis-ci.org/ruby/ruby/jobs/598491972

.travis.yml

@@ -73,6 +73,7 @@ env:
       - |-
         ${BEFORE_INSTALL}
         tool/travis_retry.sh sudo -E apt-get $travis_apt_get_options install \
+          ccache \
           gcc-8 \
           g++-8 \
           libffi-dev \
@@ -126,12 +127,6 @@ env:
     name: arm64-linux
     arch: arm64
     <<: *gcc-8
-    # arm64 build always fails with cache
-    cache:
-      ccache: false
-      directories: []
-    env:
-      - DISABLE_CCACHE=true
 
   - &jemalloc
     name: --with-jemalloc
@@ -430,14 +425,12 @@ before_script:
   - chmod -R a-w .
   - chmod -R u+w build config_1st config_2nd
   - cd build
+  - ccache --show-stats
   - |-
-    if [ "$DISABLE_CCACHE" != true ]; then
-      ccache --show-stats
-      case "$CC" in
-      gcc*)   CC="ccache $CC${GCC_FLAGS:+ }$GCC_FLAGS -fno-diagnostics-color";;
-      clang*) CC="ccache $CC${GCC_FLAGS:+ }$GCC_FLAGS -fno-color-diagnostics";;
-      esac
-    fi
+    case "$CC" in
+    gcc*)   CC="ccache $CC${GCC_FLAGS:+ }$GCC_FLAGS -fno-diagnostics-color";;
+    clang*) CC="ccache $CC${GCC_FLAGS:+ }$GCC_FLAGS -fno-color-diagnostics";;
+    esac
   - |-
     [ ! -f config.cache ] ||
     [ "$CC" = "`sed -n s/^ac_cv_prog_CC=//p config.cache`" ] ||
@@ -465,8 +458,7 @@ before_script:
         exit 1
       fi
     fi
-  - |-
-    [ "$DISABLE_CCACHE" = true ] || ccache --show-stats
+  - ccache --show-stats
   - |-
     [ -z "${GEMS_FOR_TEST}" ] ||
     $RUBY_PREFIX/bin/gem install --no-document $GEMS_FOR_TEST