Resolve "Powershell shell assumes it is ran on Windows"

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/2547

Are there points in the code the reviewer needs to double check?

No.

Testing

Tested manually on Windows 10 and Mac OS X Sierra machines.

Also automated tests pass:

gitlabs-iMac:gitlab-ci-multi-runner gitlab$ make test
make: docker: Command not found
make: docker: Command not found
make: Circular out/docker/prebuilt-x86_64.tar.xz <- executors/docker/bindata.go dependency dropped.
make: Circular out/docker/prebuilt-arm.tar.xz <- executors/docker/bindata.go dependency dropped.
# Running tests...
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner	[no test files]
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/apps/gitlab-runner-helper	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands	0.165s	coverage: 15.1% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands/helpers	1.045s	coverage: 84.9% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/common	0.034s	coverage: 40.3% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/docker	0.034s	coverage: 29.6% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/docker/machine	0.028s	coverage: 76.3% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/kubernetes	15.036s	coverage: 65.3% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/parallels	0.018s	coverage: 3.0% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/shell	18.811s	coverage: 72.1% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/ssh	0.018s	coverage: 14.8% of statements [no tests to run]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/virtualbox	0.020s	coverage: 3.1% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers	0.013s	coverage: 53.0% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/archives	0.041s	coverage: 76.5% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/cli	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/docker	0.016s	coverage: 6.8% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/formatter	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/gitlab_ci_yaml_parser	0.017s	coverage: 14.2% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/parallels	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/prometheus	0.015s	coverage: 92.3% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/sentry	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/service	0.011s	coverage: 19.0% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/service/mocks	[no test files]
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/ssh	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/timeperiod	0.060s	coverage: 85.7% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/url	0.795s	coverage: 100.0% of statements
?   	gitlab.com/gitlab-org/gitlab-ci-multi-runner/helpers/virtualbox	[no test files]
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/network	5.816s	coverage: 83.2% of statements
ok  	gitlab.com/gitlab-org/gitlab-ci-multi-runner/shells	0.052s	coverage: 28.2% of statements

Does this MR meet the acceptance criteria?

  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #2547