Use uncommon build directory name

"build" is a rather common name which has the potential for file name
conflicts, as is an example below.

Processing triggers for libc-bin (2.28-7co1ba1) ...
Processing triggers for man-db (2.8.5-2bv2020dev0b1) ...
+ dpkg-checkbuilddeps
+ dpkg-parsechangelog -S Version
+ dversion=3.2.26+dfsg-3
+ checkout-tarball build
Traceback (most recent call last):
  File "/usr/bin/checkout-tarball", line 49, in <module>
    main()
  File "/usr/bin/checkout-tarball", line 25, in main
    os.makedirs(dest, exist_ok=True)
  File "/usr/lib/python3.7/os.py", line 221, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: 'build'
ERROR: Job failed: exit code 1

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>

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

@@ -38,7 +38,7 @@ dversion="$(dpkg-parsechangelog -S Version)"
 
 case "$dversion" in
     *-*)
-        checkout-tarball build
+        checkout-tarball _build
         ;;
     *)
         ;;
@@ -51,8 +51,8 @@ if [ -z "${RELEASE}" ]; then
 fi
 
 gbp buildpackage  --git-ignore-branch \
-                  --git-export-dir=build \
-                  --git-tarball-dir=build \
+                  --git-export-dir=_build \
+                  --git-tarball-dir=_build \
                   --git-ignore-new \
                   --git-postexport="${POSTHOOK}" \
                   -uc -us \

apertis-package-source-builder/overlay/usr/bin/ci-obs-upload

@@ -41,7 +41,7 @@ release=${CI_COMMIT_REF_NAME#*/}
 
 
 prj="apertis:$release:$component:snapshots"
-osc dput "$prj" build/*.dsc
+osc dput "$prj" _build/*.dsc
 if [ -n "${RELEASE}" ];
 then
   osc submitrequest --yes --cleanup -m "Released" ${prj} ${pkg} "apertis:$release:$component"