Enable BuildKit on build Gitpod worksapce image

What does this Merge Request do and why?

Enables BuildKit on build Gitpod worksapce image (in build-gitpod-workspace-image job only).

1. Makes it visible which stage takes time to enable us to optimize the pipeline more.
2. Make the build time 2% faster than non-BuildKit (see the below Table).

before	after	diff
3128 sec	3064 sec	-2.05%

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Related to #1219 (closed)