According to the documentation[1], the path should be absolute.
This commit fixes an error while using the script under a
debian-bullseye system:

depot_tools/bootstrap_python3: line 28: cipd: command not found
depot_tools/bootstrap_python3: line 32: bootstrap-2@3.8.10.chromium.20_bin/python3/bin/python3: No such file or directory
cat: depot_tools/python_bin_reldir.txt: No such file or directory

[1] https://chromium.googlesource.com/chromium/src.git/+/refs/heads/main/docs/linux/build_instructions.md#install



Signed-off-by: Mylène Josserand <mylene.josserand@collabora.com>