Put the apt-cache in build/

Like with ccache, we want to make sure the git checkout is considered
clean during build.

.gitlab/ci/jobs/macos-x86_64.yml

@@ -14,7 +14,7 @@ osxcross x86_64:
 
     - key: apt-$CI_JOB_IMAGE
       paths:
-        - apt-cache
+        - build/apt-cache
       unprotect: true
 
     - key: vcpkg-root

.gitlab/ci/jobs/windows-x86.yml

@@ -16,7 +16,7 @@ Windows x86:
 
     - key: apt-$CI_JOB_IMAGE
       paths:
-        - apt-cache
+        - build/apt-cache
       unprotect: true
 
     - key: vcpkg-root

.gitlab/ci/templates/srb2ci.yml

@@ -12,7 +12,7 @@
 
     - key: apt-$CI_JOB_IMAGE
       paths:
-        - apt-cache
+        - build/apt-cache
       unprotect: true
 
   before_script:
@@ -37,7 +37,7 @@
     - - |
           # apt_conf
           echo -e "\e[0Ksection_start:`date +%s`:apt_conf[collapsed=true]\r\e[0KSetting up APT conf"
-      - export APT_CACHE_DIR=`pwd`/apt-cache
+      - export APT_CACHE_DIR=$CI_PROJECT_DIR/build/apt-cache
       - mkdir --parents --verbose $APT_CACHE_DIR/partial/
       - touch /etc/apt/apt.conf.d/99build
       - |