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. 🏂🏻

cli/agent_test.go

@@ -146,8 +146,10 @@ func TestWorkspaceAgent(t *testing.T) {
 		}).WithAgent().Do()
 
 		coderURLEnv := "$CODER_URL"
+		headerCmd := "printf X-Process-Testing=very-wow-" + coderURLEnv + "'\\r\\n'X-Process-Testing2=more-wow"
 		if runtime.GOOS == "windows" {
 			coderURLEnv = "%CODER_URL%"
+			headerCmd = "echo X-Process-Testing=very-wow-" + coderURLEnv + "& echo X-Process-Testing2=more-wow"
 		}
 
 		logDir := t.TempDir()
@@ -159,7 +161,7 @@ func TestWorkspaceAgent(t *testing.T) {
 			"--log-dir", logDir,
 			"--agent-header", "X-Testing=agent",
 			"--agent-header", "Cool-Header=Ethan was Here!",
-			"--agent-header-command", "printf X-Process-Testing=very-wow-"+coderURLEnv+"'\\r\\n'X-Process-Testing2=more-wow",
+			"--agent-header-command", headerCmd,
 			"--socket-path", testutil.AgentSocketPath(t),
 		)
 		clitest.Start(t, agentInv)