Use child dir for branch taken

[andrewwdye]

Why are the changes needed?

We reuse the same data directory for the branch node and the branch taken (child).

What changes were proposed in this pull request?

This change adds an additional set of directories to decouple these. It makes sure to copy the child output up to the branch node output like we do in subworkflow

How was this patch tested?

Example

from dataclasses import dataclass
from flytekit import conditional, task, workflow

@dataclass
class MyData:
    foo: str

@workflow
def root_wf(data: MyData) -> str:
    return sub_wf(data=data)

@workflow
def sub_wf(data: MyData) -> str:
    check = always_true()
    return conditional("decision").if_(check.is_true()).then(conditional_wf(data=data)).else_().fail("not done")

@task
def always_true() -> bool:
    return True

@workflow
def conditional_wf(data: MyData) -> str:
    return done(data)

@task
def done(data: MyData) -> str:
    return f"done ({data.foo})"

• Add unittests
• Run locally using sandbox and verify behavior with working example. Inspect paths

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 36.97%. Comparing base (27b2f3a) to head (97e17ab).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...tepropeller/pkg/controller/nodes/branch/handler.go	40.00%	9 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6120      +/-   ##
==========================================
- Coverage   36.97%   36.97%   -0.01%     
==========================================
  Files        1318     1318              
  Lines      132506   132516      +10     
==========================================
+ Hits        48997    49000       +3     
- Misses      79254    79258       +4     
- Partials     4255     4258       +3     

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.05% <ø> (ø)
unittests-flytecopilot	30.99% <ø> (ø)
unittests-flytectl	62.29% <ø> (ø)
unittests-flyteidl	7.23% <ø> (ø)
unittests-flyteplugins	53.85% <ø> (ø)
unittests-flytepropeller	42.59% <40.00%> (-0.01%)	⬇️
unittests-flytestdlib	55.18% <ø> (+0.01%)	⬆️

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