fix: change $sa to $existingLogin

[jkrilov]

Description

Changed incorrect references of $sa variable to $existingLogin in Login.ps1. Bug described in #159

How Has This Been Tested?

N/A

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue) - Fixes #
• [ ] New feature (non-breaking change which adds functionality)

Checklist:

• [x] I have read/followed the CONTRIBUTING document.
• [x] I have read/followed the PR Quick Start Guide
• [x] I have added tests to cover my changes or they are N/A.
• [x] I have added a changelog fragment describing the changes.
• [ ] New module options/parameters include a version_added property.

[codecov-commenter]

Codecov Report

Base: 93.78% // Head: 93.78% // No change to project coverage :thumbsup:

Coverage data is based on head (c64afd7) compared to base (dbb925d). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #161 +/- ## ======================================= Coverage 93.78% 93.78% ======================================= Files 64 64 Lines 2188 2188 ======================================= Hits 2052 2052 Misses 136 136 ``` | [Impacted Files](https://codecov.io/gh/lowlydba/lowlydba.sqlserver/pull/161?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+McCall) | Coverage Δ | | |---|---|---| | [plugins/modules/login.ps1](https://codecov.io/gh/lowlydba/lowlydba.sqlserver/pull/161/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+McCall#diff-cGx1Z2lucy9tb2R1bGVzL2xvZ2luLnBzMQ==) | `96.59% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+McCall). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+McCall)

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[lowlydba]

Sorry for the time to merge on this, the Windows workflows are a bit flaky. Going to increase the reliability via #165

[jkrilov]

Not a problem. Curious if this has any impact on the other bug I just opened.