fix: Set .Requirements.repository by default to "nexus"

[abayer]

Submitter checklist

• Change is code complete and matches issue description.
• Change is covered by existing or new tests.

Description

The helm charts in jenkins-x-boot-config need to check this value a bunch of times, so it'd be awkward to tweak them to always do a hasKey check first. So let's set this by default to "nexus" - can't go with "" as the default because the field is omitempty.

Fixes version stream BDD failures like:

STEP: install-jenkins-x command: /bin/sh -c ** step helm apply --boot --remote --name jenkins-x --provider-values-dir ../kubeProviders in dir: /workspace/source/boot-source/env
Modified file /workspace/source/boot-source/env/Chart.yaml to set the chart to version 1
error: generating values.yaml for tree from /tmp/**-helm-apply-164375805/env: failed to execute Secrets template: /tmp/**-helm-apply-164375805/env/bucketrepo/values.tmpl.yaml: template: values.tmpl.yaml:1:23: executing "values.tmpl.yaml" at <.Requirements.repository>: map has no entry for key "repository"
error: failed to interpret pipeline file jenkins-x.yml: failed to run '/bin/sh -c ** step helm apply --boot --remote --name jenkins-x --provider-values-dir ../kubeProviders' command in directory 'env', output: ''
ERROR: Error: Command failed  ** boot -b
error: exit status 1

Special notes for the reviewer(s)

Which issue this PR fixes

fixes #6011

[abayer]

/test integration

[rawlingsj]

/lgtm

[jenkins-x-bot]

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[abayer]

/retest