Add option to inject EFFECTIVE_VERSION build-arg

[oliver-goetz]

/kind bug

What this PR does / why we need it:
Add option to inject EFFECTIVE_VERSION build-arg that gardener/gardener binaries contain the right version info.

Manual test for a PR presubmit job:
https://prow.gardener.cloud/view/gs/gardener-prow/pr-logs/pull/gardener_gardener/5779/pull-gardener-publish-test-images/1513833918111944704

➜  ~ docker run --rm eu.gcr.io/gardener-project/gardener/apiserver:v1.45.0-dev-4f5dd702ed31a14abc026d3bce8381cbddbe97ed --version                                                                                                                (gardener-prow-trusted/default)
Unable to find image 'eu.gcr.io/gardener-project/gardener/apiserver:v1.45.0-dev-4f5dd702ed31a14abc026d3bce8381cbddbe97ed' locally
v1.45.0-dev-4f5dd702ed31a14abc026d3bce8381cbddbe97ed: Pulling from gardener-project/gardener/apiserver
5758d4e389a3: Already exists
317fe4fc145e: Pull complete
a92bd55327e9: Pull complete
Digest: sha256:d402045447d0aad07b9bab49f30e02099cd34242d59cdb54f00c0aae5f38799f
Status: Downloaded newer image for eu.gcr.io/gardener-project/gardener/apiserver:v1.45.0-dev-4f5dd702ed31a14abc026d3bce8381cbddbe97ed
Gardener v1.45.0-dev-4f5dd702ed31a14abc026d3bce8381cbddbe97ed

Manual test for a postsubmit job:
https://prow.gardener.cloud/view/gs/gardener-prow/logs/post-gardener-build-dev-images/1513834382115213312

➜  ~ docker run --rm eu.gcr.io/gardener-project/gardener/apiserver:v1.45.0-dev-f5882800d5821bcc18198da96324bbe013bb1afe --version                                                                                                                (gardener-prow-trusted/default)
Unable to find image 'eu.gcr.io/gardener-project/gardener/apiserver:v1.45.0-dev-f5882800d5821bcc18198da96324bbe013bb1afe' locally
v1.45.0-dev-f5882800d5821bcc18198da96324bbe013bb1afe: Pulling from gardener-project/gardener/apiserver
5758d4e389a3: Already exists
fd2d5834177c: Pull complete
1ab5ad74e424: Pull complete
Digest: sha256:89671e05c1d5e1b91b58edc898a9c90a1742cd30ef21453b6056ad2844f1b04b
Status: Downloaded newer image for eu.gcr.io/gardener-project/gardener/apiserver:v1.45.0-dev-f5882800d5821bcc18198da96324bbe013bb1afe
Gardener v1.45.0-dev-f5882800d5821bcc18198da96324bbe013bb1afe

[oliver-goetz]

/cla

[gardener-prow]

Successfully reached out to cla-assistant.io to initialize recheck of PR #190

[timebertt]

/cla

[gardener-prow]

Successfully reached out to cla-assistant.io to initialize recheck of PR #190

[timebertt]

/override license/cla

[gardener-prow]

@timebertt: Overrode contexts on behalf of timebertt: license/cla

In response to this:

/override license/cla

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[timebertt]

/lgtm

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: timebertt

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[timebertt]

ref #180