[backend]: Determine cache scope for pipeline-root and namespace

[zijianjoy]

How should cache work when everything is same except the pipeline_root?
For example,
pipeline #1 uses minio://xxxx as the pipeline_root
pipeline #2 uses gs://xxxx as the pipeline_root
other configs of pipeline#1 and pipeline#2 are same.
Now pipeline #2 will hit the cache of pipeline #1 but it won't use the pipeline root under gs://xxxx, it will use the pipeline root under minio://xxxx as what pipeline#1 does.

Discussed with @Bobgy @IronPan, this is the expected behavior.

EDIT by @Bobgy :
However, before implementing support for downloading artifacts not from the pipeline root, the pipeline still breaks.
TODOs:

• add this as a known caveat
• when supporting importer, we need to do the same fix

EDIT by @capri-xiyue
For shared pipelines, cross-namespace cache hit is not allowed
TODOs:

• add namespace as part of the cache scope
  Originally posted by @capri-xiyue in [release] 1.7.0 tracker #5779 (comment)

[zijianjoy]

/assign @Bobgy
/assign @capri-xiyue

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