fix: never attempt to call builtins.storePath in pure evaluation mode

After 5a16547d46, I started getting errors like https://github.com/nix-community/home-manager/issues/2409.

@moduon MT-1075

.github/workflows/flakehub-publish.yml

@@ -0,0 +1,18 @@
+name: "Publish to FlakeHub"
+on:
+  push:
+    tags:
+      - "v*.*.*"
+jobs:
+  flakehub-publish:
+    runs-on: "ubuntu-latest"
+    permissions:
+      id-token: "write"
+      contents: "read"
+    steps:
+      - uses: "actions/checkout@v3"
+      - uses: "DeterminateSystems/nix-installer-action@main"
+      - uses: "DeterminateSystems/flakehub-push@main"
+        with:
+          name: "edolstra/flake-compat"
+          visibility: "public"

README.md

@@ -2,25 +2,10 @@
 
 ## Usage
 
-> [!WARNING]
-> During the Lix private beta period during which Forgejo is private, this
-> requires configuring netrc in Lix for the tarball download to work.
->
-> Your netrc should look something like so:
-> ```
-> machine git.lix.systems login YOUR-USERNAME password SOME-PERSONAL-ACCESS-TOKEN-REPO-READ
-> ```
->
-> We are terribly sorry for the UX for this being very bad
-> ([issue](https://git.lix.systems/lix-project/lix/issues/254)).
-
 To use, add the following to your `flake.nix`:
 
-<!-- FIXME: this can use the standard non-api archive url when we are
-un-privated -->
-
 ```nix
-inputs.flake-compat.url = "https://git.lix.systems/api/v1/repos/lix-project/flake-compat/archive/main.tar.gz";
+inputs.flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";
 ```
 
 Afterwards, create a `default.nix` file containing the following:
@@ -30,7 +15,7 @@ Afterwards, create a `default.nix` file containing the following:
   (
     let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
     fetchTarball {
-      url = lock.nodes.flake-compat.locked.url;
+      url = lock.nodes.flake-compat.locked.url or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
       sha256 = lock.nodes.flake-compat.locked.narHash;
     }
   )

default.nix

@@ -73,7 +73,7 @@ let
         shortRev = builtins.substring 0 7 info.rev;
       }
     else
-      # FIXME: add Mercurial inputs.
+      # FIXME: add Mercurial, tarball inputs.
       throw "flake input has unsupported input type '${info.type}'";
 
   callFlake4 = flakeSrc: locks:
@@ -109,9 +109,7 @@ let
           # Massage `src` into a store path.
           if builtins.isPath src
           then
-            if dirOf (toString src) == builtins.storeDir
-              # `builtins.storePath` is not available in pure-eval mode.
-              && builtins ? currentSystem
+            if builtins ? currentSystem && dirOf (toString src) == builtins.storeDir
             then
               # If it's already a store path, don't copy it again.
               builtins.storePath src