Agent install tests failing on `main`

Seen in https://buildkite.com/elastic/elastic-stack-installers/builds/5429#01903577-fb5d-45f5-8f2f-2ca2bf7a942e via an unrelated PR: https://github.com/elastic/elastic-stack-installers/pull/276

Error:

```
{"log.level":"info","@timestamp":"2024-06-20T12:08:03.107Z","log.origin":{"file.name":"cmd/enroll_cmd.go","file.line":518},"message":"Starting enrollment to URL: https://placeholder:443/","log":{"source":"elastic-agent"},"ecs.version":"1.6.0"}
[-] Can be installed and uninstalled via MSI, with installargs, in Fleet mode 68.88s (68.87s|9ms)
[0] Expected $true, but got $false.
at Has-AgentFleetEnrollmentAttempt | Should -BeTrue, C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:53
at , C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:53
at Assert-AgentHealthy, C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:119
at , C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:126
[1] Expected $false, because The agent should have been cleaned up already, but got $true.
at Is-AgentBinaryPresent | Should -BeFalse -Because "The agent should have been cleaned up already", C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:72
at Check-AgentRemnants, C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:72
at , C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\msi.tests.ps1:109
WARNING: Found Agent remnants between tests. Removing Agent.
Open file may block uninstall \Device\HarddiskVolume3\Program Files\Elastic\Agent\data\elastic-agent-8.15.0-SNAPSHOT-33598b\logs\elastic-agent-20240620.ndjson with PID 840 opened by elastic-agent
VERBOSE: Invoking msiexec.exe /x {E550A894-5C44-5BEF-9967-A2CD4B022161} /qn /l*v "C:\buildkite-agent\builds\bk-agent-prod-gcp-1718883984823248883\elastic\elastic-stack-installers\src\agent-qa\logs\20240620-120807-x.log"
```

Link to CI logs: https://buildkite.com/elastic/elastic-stack-installers/builds/5429#01903577-fb5d-45f5-8f2f-2ca2bf7a942e/3842-4533