Skip workspace-wide `pnpm install` when JS deps are already installed.

Nixpkgs installs the filtered admin UI workspace via pnpmConfigHook.
A second install walks every workspace member and fails offline.

--- a/crates/build/src/lib.rs
+++ b/crates/build/src/lib.rs
@@ -92,39 +92,43 @@
     Ok("TRUE") | Ok("true") | Ok("1")
   );
 
-  // Note that `--frozen-lockfile` and `-ignore-workspace` are practically exclusive
-  // Because ignoring the workspace one, will require to create a new lockfile.
-  let out_dir = std::env::var("OUT_DIR").unwrap();
-  let build_result = if out_dir.contains("target/package") {
-    // When we build cargo packages, we cannot rely on the workspace itself and prior installs.
-    pnpm_run(&["--dir", &path, "install", "--ignore-workspace"])
-  } else {
-    // `trailbase-assets` and `trailbase-js` both build JS packages. We've seen issues with
-    // parallel installs in the past. Our current approach is to recommend installing workspace
-    // JS deps upfront in combination with `--prefer-offline`. We used to use plain `--offline`,
-    // however this adds an extra mandatory step when vendoring trailbase for framework use-cases.
-    let args = if strict_offline {
-      ["--dir", &path, "install", "--frozen-lockfile", "--offline"]
+  // Packagers such as Nixpkgs install JS deps before `cargo build`.
+  // Skip the extra workspace-wide install in that case.
+  if std::env::var_os("TRAILBASE_SKIP_PNPM_INSTALL").is_none() {
+    // Note that `--frozen-lockfile` and `-ignore-workspace` are practically exclusive
+    // Because ignoring the workspace one, will require to create a new lockfile.
+    let out_dir = std::env::var("OUT_DIR").unwrap();
+    let build_result = if out_dir.contains("target/package") {
+      // When we build cargo packages, we cannot rely on the workspace itself and prior installs.
+      pnpm_run(&["--dir", &path, "install", "--ignore-workspace"])
     } else {
-      [
-        "--dir",
-        &path,
-        "install",
-        "--prefer-frozen-lockfile",
-        "--prefer-offline",
-      ]
+      // `trailbase-assets` and `trailbase-js` both build JS packages. We've seen issues with
+      // parallel installs in the past. Our current approach is to recommend installing workspace
+      // JS deps upfront in combination with `--prefer-offline`. We used to use plain `--offline`,
+      // however this adds an extra mandatory step when vendoring trailbase for framework use-cases.
+      let args = if strict_offline {
+        ["--dir", &path, "install", "--frozen-lockfile", "--offline"]
+      } else {
+        [
+          "--dir",
+          &path,
+          "install",
+          "--prefer-frozen-lockfile",
+          "--prefer-offline",
+        ]
+      };
+      let build_result = pnpm_run(&args);
+      if build_result.is_err() {
+        error!(
+          "`pnpm {}` failed. Make sure to install all JS deps first using `pnpm install`",
+          args.join(" ")
+        )
+      }
+      build_result
     };
-    let build_result = pnpm_run(&args);
-    if build_result.is_err() {
-      error!(
-        "`pnpm {}` failed. Make sure to install all JS deps first using `pnpm install`",
-        args.join(" ")
-      )
-    }
-    build_result
-  };
 
-  let _ = build_result?;
+    let _ = build_result?;
+  }
 
   // Enable source maps for debug builds:
   // let build_output = pnpm_run(&["--dir", &path, "build", "--sourcemap", "inline"]);