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>