[S3 Cache] Allow restoring cache to absolute file path rather than within the build workspace

[ecrupper]

Description

Currently, you can't specify an absolute path to restore cached objects. Instead, all restorations are within the pwd, which happens to be the $VELA_BUILD_WORKSPACE.

Value

There is value in being able to leverage paths such as vela/parameters if one were to use a build plugin that could intake cached objects.

Definition of Done

A user can specify an absolute path for restore objects.

Effort (Optional)

Low

[wass3rw3rk]

just leaving a note: there is a case to be made to be more restrictive and only allow caching (build and restore) from within the build workspace as vela/parameters and such could contain sensitive values that would be shipped off to cache store.