ci: add Git usr/bin to PATH on Windows (#25939)

## Summary

Fixes all 9 Windows CI test failures caused by the mise CI refactor
(`fe257666d7`, PR #25727).

### Root cause

`jdx/mise-action` exports `Path` (Windows convention) via `GITHUB_ENV`.
Bash on Windows maintains its own `PATH`. When Go's `os.Environ()`
returns both, `cmd.exe` subprocesses non-deterministically pick the
MSYS-translated `PATH` (forward slashes), causing Windows executables
(`printf`, `powershell.exe`, `cmd.exe`) to be unresolvable.

These failures only appeared on `main` (where `-count=1` forces real
test execution) and were masked on PRs by Go test cache.

### Fixes applied

**CI (`setup-mise` action)**:
- Write both `Path` and `PATH` to `GITHUB_ENV` with Git usr/bin
prepended

**Code (`cli/root.go`)**:
- Add `appendAndDedupEnv` helper that deduplicates case-insensitive env
vars on Windows, preferring native Windows paths (backslashes) over MSYS
paths

**Code (`cli/configssh_windows.go`)**:
- Use absolute paths for `powershell.exe` and `cmd.exe` in the SSH
config `Match exec` escape function, avoiding PATH resolution entirely

**Tests**:
- Switch `--header-command` tests from `printf` to `echo` (cmd.exe
builtin) for reliable cross-platform execution
- Add env dedup in `Test_sshConfigMatchExecEscape` for subprocess PATH
consistency

Fixes coder/internal#1556, coder/internal#1558, coder/internal#1559

> 🤖 Generated by Coder agent, will be reviewed by @mafredri. 🏂🏻

.github/actions/setup-mise/action.yml

@@ -166,3 +166,23 @@ runs:
         mise_dir: ${{ steps.mise-data-dir.outputs.path }}
         install_args: ${{ steps.cache-key.outputs.install-args }}
         cache: "false"
+
+    - name: Ensure Git usr/bin is in PATH (Windows)
+      if: runner.os == 'Windows'
+      shell: pwsh
+      # jdx/mise-action exports "Path" via GITHUB_ENV which may
+      # collide with bash's "PATH". Ensure Git usr/bin is present
+      # and remove any duplicate Path/PATH entries from GITHUB_ENV
+      # by writing both forms.
+      run: | # zizmor: ignore[github-env]
+        $gitdir = "C:\Program Files\Git\usr\bin"
+        $current = $env:Path
+        if ($current -notlike "*$gitdir*") {
+          $current = "$gitdir;$current"
+        }
+        # Write both Path and PATH to GITHUB_ENV so that both
+        # cmd.exe (uses Path) and bash/Go (uses PATH) see the
+        # same value including Git usr/bin.
+        "Path=$current" | Out-File -Append -FilePath $env:GITHUB_ENV -Encoding utf8
+        "PATH=$current" | Out-File -Append -FilePath $env:GITHUB_ENV -Encoding utf8
+