Repository context of subinstallations

[robertgraeff]

What this PR does / why we need it:

The repository context of an installation can be specified in different places:

1. in the Installation
2. in the Context referenced by the Installation
3. in the OCM config

In a scenario where the repository context was in place 3, subinstallations contained a repository context in place 1, and moreover this was the wrong repository context.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

- Bugfix concerning repository contexts of subinstallations.

[gardener-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[gardener-robot]

@robertgraeff You need rebase this pull request with latest master branch. Please check.

[gardener-prow]

@robertgraeff: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-landscaper-check-renovate-config	df31b9c	link	true	/test pull-landscaper-check-renovate-config

Full PR test history. Your PR dashboard. Command help for this repository.
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