build: update Windows symlink scripts

[bisubus]

Fixes packages\upgrade\static\src symlink.

Closes #16760

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

See #16760

What is the new behavior?

packages\upgrade\static\src symbolic link behaves as expected on Windows.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[gkalpak]

@IgorMinar, @mhevery, could we get a LGTM here, please 😁

[gkalpak]

/ping @mhevery, @IgorMinar

[gkalpak]

🎉

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