Add system install for GitLab's runner

[NikhilSharmaWe]

Description

This PR add new option for system install for installing gitlab-runner.

Motivation and Context

• I have raised an issue to propose this change, which has been given a label of design/approved by a maintainer (required)
  Fixes: Add GitLab's runner to arkade system install #907

How Has This Been Tested?

I test it by running the command arkade system install gitlab-runner locally. This command downloads the gitlab binary file in the dir path specified by the --path flag.

> ./arkade system install gitlab-runner
Installing GitLab Runner to $HOME/gitlab-runner
Installing version: latest for: amd64
Downloading from: https://gitlab-runner-downloads.s3.amazonaws.com/latest/binaries/gitlab-runner-linux-amd64
57.62 MiB / 57.62 MiB [--------------------------------------------------------------------------------------------] 100.00%
Downloaded to: /tmp/gitlab-runner-linux-amd64
Binary file copied to: /home/nikhilsharmawe/gitlab-runner

• Demo: https://asciinema.org/a/NpLOebT2bxyNMj1b6nED4RGR5

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

• I have updated the list of tools in README.md if (required) with ./arkade get -o markdown
• I have updated the list of apps in README.md if (required) with ./arkade install --help

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

• I have tested this on arm, or have added code to prevent deployment

[alexellis]

Hey this is great thank you for doing it so quickly.

Could you download the Arm64 and AMD64 binaries and show using file that they have the correct architecture?

You can't use this script, but it may show you what I mean - https://github.com/alexellis/arkade/blob/master/hack/test-tool.sh

[NikhilSharmaWe]

@alexellis
I have added a demo in the PR description regarding this.

[alexellis]

That demo looks great. Thank you for adding the variations and output from file