fix: check if user passes absolute path to prune (#4891)

### Description

As discovered in #4885, we forcibly join `--out-dir` to the repo root
even if it's absolute. This PR adds a check that skips the join if the
user passes an absolute system path.

### Testing Instructions

Added failing integration test

cli/internal/prune/prune.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/vercel/turbo/cli/internal/cmdutil"
@@ -69,7 +70,12 @@ func (p *prune) prune(opts *turbostate.PrunePayload, packageManagerName string)
 	if err != nil {
 		return errors.Wrap(err, "could not construct graph")
 	}
-	outDir := p.base.RepoRoot.UntypedJoin(opts.OutputDir)
+	var outDir turbopath.AbsoluteSystemPath
+	if filepath.IsAbs(opts.OutputDir) {
+		outDir = turbopath.AbsoluteSystemPathFromUpstream(opts.OutputDir)
+	} else {
+		outDir = p.base.RepoRoot.UntypedJoin(opts.OutputDir)
+	}
 	fullDir := outDir
 	if opts.Docker {
 		fullDir = fullDir.UntypedJoin("full")

turborepo-tests/integration/tests/prune/out_dir.t

@@ -0,0 +1,24 @@
+Setup
+  $ . ${TESTDIR}/../../../helpers/setup.sh
+  $ . ${TESTDIR}/../_helpers/setup_monorepo.sh $(pwd) monorepo_with_root_dep
+
+Test that absolute paths can be passed as out-dir
+  $ TMPFILE=$(mktemp)
+  $ ${TURBO} prune --scope=web --out-dir=${TMPFILE}
+  Generating pruned monorepo for web in .* (re)
+   - Added shared
+   - Added util
+   - Added web
+  $ cat ${TMPFILE}/package.json
+  {
+    "devDependencies": {
+      "util": "workspace:*"
+    },
+    "name": "monorepo",
+    "packageManager": "pnpm@7.25.1",
+    "pnpm": {
+      "patchedDependencies": {
+        "is-number@7.0.0": "patches/is-number@7.0.0.patch"
+      }
+    }
+  }