Use path instead of filepath

Fix an issue when using bash on Windows system leads to creating invalid path where are stored temporary files.

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1233

mentioned in issue #1233 (closed)

Status changed to merged

mentioned in issue #1515

@tmaczukin Got the same using bleeding edge executable from master.

Running with gitlab-ci-multi-runner 1.8.0~beta.3.g51eb636 (51eb636)
Using Shell executor...
Running on GIMLI...
bash: line 54: /d/gitlab-ci-runner/D:/gitlab-ci-runner/builds/88a9dbf3/0/gitlab/duopo/mapa-franquias.tmp/GIT_SSL_CAINFO: No such file or directory
ERROR: Build failed: exit status 1

Mentioned in issue #1515