ci: Don't build sphinx documentation on centos

The centos docker image lacks the sphinx builder so we disable building
sphinx docs for these jobs.

Fixes #21580

.gitlab/gen_ci.hs

@@ -357,6 +357,9 @@ distroVariables Alpine = mconcat
     -- (see Note [Object unloading]).
   , "BROKEN_TESTS" =: "encoding004 T10458 ghcilink002 linker_unload_native"
   ]
+distroVariables Centos7 = mconcat [
+  "HADRIAN_ARGS" =: "--docs=no-sphinx"
+  ]
 distroVariables Fedora33 = mconcat
   -- LLC/OPT do not work for some reason in our fedora images
   -- These tests fail with this error: T11649 T5681 T7571 T8131b

.gitlab/jobs.yaml

@@ -781,6 +781,7 @@
       "BIN_DIST_NAME": "ghc-x86_64-linux-centos7-validate",
       "BUILD_FLAVOUR": "validate",
       "CONFIGURE_ARGS": "",
+      "HADRIAN_ARGS": "--docs=no-sphinx",
       "TEST_ENV": "x86_64-linux-centos7-validate",
       "XZ_OPT": "-9"
     }
@@ -2230,6 +2231,7 @@
       "BIN_DIST_NAME": "ghc-x86_64-linux-centos7-release",
       "BUILD_FLAVOUR": "release",
       "CONFIGURE_ARGS": "",
+      "HADRIAN_ARGS": "--docs=no-sphinx",
       "IGNORE_PERF_FAILURES": "all",
       "TEST_ENV": "x86_64-linux-centos7-release",
       "XZ_OPT": "-9"
@@ -3080,6 +3082,7 @@
       "BIN_DIST_NAME": "ghc-x86_64-linux-centos7-validate",
       "BUILD_FLAVOUR": "validate",
       "CONFIGURE_ARGS": "",
+      "HADRIAN_ARGS": "--docs=no-sphinx",
       "TEST_ENV": "x86_64-linux-centos7-validate"
     }
   },