test_install_ps1_json_failure_frame runs a full `install.ps1 -Stage repository
-Json` under Windows PowerShell with a 30 s timeout. On the windows-arm64 lane
that run takes 25.0 s when green and 68.8 s when slow, so it times out
intermittently (twice on PR #130298, a gateway-only change). The same file
invocation in test_install_ps1_staged_git uses 240 s; use that budget here and
for staged_git's dot-sourced Ensure-Git probe, which parses the same script under
the same 30 s limit.
Also read the preserved user file with utf-8-sig, the BOM-tolerant read the
Windows-footgun lint requires for this file.