restore manager bootstrap creds for subscription ID

[nojnhuh]

What type of PR is this?
/kind bug

What this PR does / why we need it:

When we required AzureClusterIdentity starting in #3793, we removed the authentication environment variables from the CAPZ controller manager Pod, including AZURE_SUBSCRIPTION_ID. That value cannot be defined in an AzureClusterIdentity and is instead an optional field on AzureCluster and AzureManagedControlPlane. That "optional" qualifier requires that users are able to specify a subscription ID some other way. This PR restores the AZURE_SUBSCRIPTION_ID variable for the controller manager Pod and can be set when CAPZ is installed the same way it could have been in CAPZ v1.10 and earlier.

This is a near-term, backwards compatible fix to help us flesh out how we might want to modify the subscriptionID field longer-term in #4612.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #4557

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

A regression making `spec.subsciptionID` required on AzureCluster and AzureManagedControlPlane has been fixed. Users can specify a global subscription ID with the `AZURE_SUBSCRIPTION_ID_B64` environment variable when CAPZ is installed as was possible in CAPZ v1.10 and earlier.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.46%. Comparing base (cdf5992) to head (8c3deed).
Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4784   +/-   ##
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  Coverage   62.46%   62.46%           
=======================================
  Files         196      196           
  Lines       16180    16180           
=======================================
  Hits        10107    10107           
  Misses       5333     5333           
  Partials      740      740           

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[mboersma]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: a65ca719f2986fe1d318dacfdfe48d85c2e12693

[jackfrancis]

/assign @willie-yao

[willie-yao]

/lgtm

[jackfrancis]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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