gitlab-ci: pxz is unavailable on CentOS 7

Fall back to xz

(cherry picked from commit 8565f808)

.gitlab-ci.yml

@@ -713,6 +713,8 @@ release-x86_64-linux-centos7:
     BUILD_SPHINX_PDF: "NO"
     TEST_ENV: "x86_64-linux-centos7"
     BIN_DIST_PREP_TAR_COMP: "ghc-x86_64-centos7-linux.tar.xz"
+    # pxz is not available on centos7 for reasons I can't understand
+    XZ: "xz"
   only:
     - tags
   cache:

.gitlab/prepare-system.sh

@@ -13,6 +13,7 @@ if [[ -z ${BUILD_SPHINX_HTML:-} ]]; then BUILD_SPHINX_HTML=YES; fi
 if [[ -z ${BUILD_SPHINX_PDF:-} ]]; then BUILD_SPHINX_PDF=YES; fi
 if [[ -z ${INTEGER_LIBRARY:-} ]]; then INTEGER_LIBRARY=integer-gmp; fi
 if [[ -z ${BUILD_FLAVOUR:-} ]]; then BUILD_FLAVOUR=perf; fi
+if [[ -z ${XZ:-} ]]; then XZ=pxz; fi
 
 if [[ -z ${XZ:-} ]]; then
   if which pxz; then