local backend test shells if unknown (#5570)

currently if we don't know the shell we just assume posix.
this adds a small test, to ensure it is and fail gracefully before doing weird stuff.

## Test Conf

```yaml
skip_clone: true
steps:
  build:
    image: "true"
    commands:
      - echo "building..."
```

pipeline/backend/local/command_test.go

@@ -16,6 +16,7 @@ package local
 
 import (
 	"os"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -137,3 +138,35 @@ echo new line
 		assert.Equal(t, "-e", args[0])
 	})
 }
+
+func TestProbeShellIsPosix(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("skipping posix shell tests on non-linux system")
+	}
+
+	t.Run("valid posix shells", func(t *testing.T) {
+		err := probeShellIsPosix("sh")
+		assert.NoError(t, err)
+	})
+
+	t.Run("invalid shell", func(t *testing.T) {
+		err := probeShellIsPosix("nonexistentshell12345")
+		if assert.ErrorIs(t, err, &ErrNoPosixShell{}) {
+			assert.Equal(t,
+				`Shell "nonexistentshell12345" was assumed to be a Posix shell, but test failed: exec: "nonexistentshell12345": executable file not found in $PATH
+(if you want support for it, please open an issue)`,
+				err.Error())
+		}
+	})
+
+	t.Run("non-posix shell", func(t *testing.T) {
+		// nologin won't understand posix syntax
+		err := probeShellIsPosix("true")
+		if assert.ErrorIs(t, err, &ErrNoPosixShell{}) {
+			assert.Equal(t,
+				`Shell "true" was assumed to be a Posix shell, but test failed: unexpected output returned: ""
+(if you want support for it, please open an issue)`,
+				err.Error())
+		}
+	})
+}