fix: allow int as job variable

What does this MR do?

This is a very pragmatic approach for fixing integer variables in exec docker. I am probably not seeing the bigger picture, CI did not run in my fork and tests did not run locally. Yaee!

aaa:
  image: alpine:latest
  script: echo $1
  variables:
    a: 1

go run . exec docker aaa
FATAL: invalid value for variable "a"
exit status 1

Why was this MR needed?

Integers are accepted as valid job variables in the "real Gitlab CI world". When trying to debug a job locally with "docker exec" an error is raised.

What's the best way to test this MR?

See above.

What are the relevant issue numbers?

#4288