[installer]: correct the starts_with validation on the config

[mrsimonemms]

Description

The validation was just being printed to terminal, not passed into the validation result. This corrects the mistake

Related Issue(s)

Fixes #7205

How to test

1. Generate a config installer init > config.yaml
2. Remove the / at the start of workspace.runtime.containerdRuntimeDir
3. Run installer validate config -c config.yaml

This should now bring the validation error out and exit with non-zero

Release Notes

[installer]: correct the starts_with validation on the config

Documentation

[codecov]

Codecov Report

Merging #7206 (91f3039) into main (300a805) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #7206   +/-   ##
=====================================
  Coverage   5.76%   5.76%           
=====================================
  Files         13      13           
  Lines       1162    1162           
=====================================
  Hits          67      67           
  Misses      1094    1094           
  Partials       1       1           

Flag	Coverage Δ
installer-raw-app	5.76% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ced67ae...91f3039. Read the comment docs.

[princerachit]

Test passed:

gitpod /workspace/gitpod/installer $ installer validate config -c config
{
  "valid": false,
  "fatal": [
    "Field 'Config.Workspace.Runtime.ContainerDRuntimeDir' must start with '/'"
  ]
}
Error: configuration invalid
Usage:
  gitpod-installer validate config [flags]

Flags:
  -h, --help   help for config

Global Flags:
  -c, --config string               path to the config file
      --debug-version-file string   path to a version manifest - not intended for production use

Error: configuration invalid
gitpod /workspace/gitpod/installer $ echo $?
1
gitpod /workspace/gitpod/installer $ 

[roboquat]

LGTM label has been added.

Git tree hash: c33d780bd0748f78045ce7674e731af85805d825

[roboquat]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: princerachit

Associated issue: #7205

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• installer/OWNERS [princerachit]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment