ci: fix Windows step shell -- use powershell, not pwsh

Real verification push (2b216d3) caught this: explicitly setting
shell: pwsh made the Windows job fail immediately with "Cannot find:
pwsh in PATH". winvm-builder only has the in-box Windows PowerShell 5.1
(powershell.exe), not PowerShell Core -- which is exactly why the original
inline build.yml steps never set `shell:` at all and just relied on the
implicit default. Switched both workflows' Windows steps to
`shell: powershell` explicitly instead of leaving it implicit, so future
edits can't reintroduce this by copying a pwsh example from elsewhere.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

.gitea/workflows/build.yml

@@ -93,7 +93,11 @@ jobs:
         uses: lukka/get-cmake@latest
 
       - name: Configure, build, test, verify
-        shell: pwsh
+        # Windows PowerShell (powershell.exe), not PowerShell Core (pwsh) --
+        # this self-hosted runner does not have pwsh installed, only the
+        # in-box Windows PowerShell 5.1 the original inline steps implicitly
+        # ran under.
+        shell: powershell
         run: ./.gitea/scripts/windows-build.ps1
 
       - name: Upload plugin