`extends` appears to be broken when used with `gitlab-runner exec`
## Summary
## Steps to reproduce
```shell
$ mkdir tmp
$ cd tmp
$ git init
# create initial comimt
$ echo hello > test
$ git add test
$ git commit -m 'initial commit'
# create gitlab-ci.yml
$ cat >.gitlab-ci.yml <<EOF
.validate-base:
script:
- echo hello
validate:
extends: .validate-base
before_script:
- echo test
EOF
$ git add .gitlab-ci.yml
$ git commit -a
$ gitlab-runner exec shell validate
FATAL: missing 'script' for job
```
## Actual behavior
It fails claiming the job is missing a `script`.
## Expected behavior
The job should run.
### Used GitLab Runner version
```
$ gitlab-runner --version
Version: 11.4.2
Git revision: cf91d5e1
Git branch: 11-4-stable
GO version: go1.8.7
Built: 2018-10-26T20:48:10+0000
OS/Arch: linux/amd64
```
issue