release-25.2: npgsql: remove setup dep on flaky Snap Store

[bghal]

Backport 1/1 commits from #169836 on behalf of @bghal.

---

The dependency caused the test to fail on multiple
branches. This change moves to Microsoft's install
script.

Epic: none
Fixes: #169533
Fixes: #169546
Fixes: #169547
Fixes: #169551
Fixes: #169576

Release note: None

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Release justification: Change to non-production test code.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[trunk-io]

😎 Merged successfully - details.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #169853 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: This PR modifies a file (npgsql.go) within a development tools directory (pkg/cmd/roachtest/), which is considered non-production. Under the current backport policy, changes to non-production code, tests, or development tools are exempt from the standard backport requirements. This specific change addresses an issue with a dependency causing the test to fail across multiple branches. The 'Release justification' line in the PR body explicitly states that the change is to non-production test code, which further supports exemption from standard backport requirements.

✅ ENGREQ Check Skipped: Test-only backport.

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