ci-buildpackage: Use a less common name for the build directory

Building packages into _build causes issues with ocaml packages: those too build into this directory by default, which is therefore removed during the clean stage.

This has been observed for ounit, pcre-ocaml and ocaml-result, and probably affects all ocaml packages.

Using a less common name for the build directory solves the issue without having to patch individual packages.

package-source-builder/overlay/usr/bin/ci-buildpackage

@@ -22,7 +22,7 @@ dversion="$(dpkg-parsechangelog -S Version)"
 
 case "$dversion" in
     *-*)
-        pristine-lfs checkout -o _build --auto
+        pristine-lfs checkout -o _apertisbuild --auto
         ;;
     *)
         ;;
@@ -37,8 +37,8 @@ fi
 export GBP_CONF_FILES=/dev/null # ignore any upstream gbp.conf shipped with the package
 
 gbp buildpackage  --git-ignore-branch \
-                  --git-export-dir=_build \
-                  --git-tarball-dir=_build \
+                  --git-export-dir=_apertisbuild \
+                  --git-tarball-dir=_apertisbuild \
                   --git-ignore-new \
                   --git-postexport="${POSTHOOK}" \
                   --git-no-create-orig \