python: avoid long paths (part 2)

python: avoid long paths (part 2)

make CEPH_BUILD_VIRTUALENV=/tmp TESTS=ceph-detect-init/run-tox.sh check

sets the CEPH_DETECT_INIT_VIRTUALENV environment variable

CEPH_BUILD_VIRTUALENV=/tmp make TESTS=ceph-detect-init/run-tox.sh check

does not because Makefile-env.am overrides it.

Do not set CEPH_BUILD_VIRTUALENV in Makefile-env.am so the variable from
the environment can be used. Require that CEPH_BUILD_VIRTUALENV has a
trailing slash so that the default is just an empty variable.

Signed-off-by: Loic Dachary <loic@dachary.org>