test(pkg): [exported_env] and [PATH] (#10095)

Demonstrate that appending to [PATH] using [exported_env] doesn't allow
us to access additional binaries.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

test/blackbox-tests/test-cases/pkg/setenv-bin-dir.t

@@ -0,0 +1,42 @@
+We set the PATH with (exported_env ..) and this should be reflected when
+looking up binaries in the workspace.
+
+  $ . ./helpers.sh
+
+  $ make_lockdir
+
+  $ bin=foobarbin
+
+  $ mkdir _bin
+  $ cat >_bin/$bin <<EOF
+  > #!/usr/bin/env sh
+  > echo foo
+  > EOF
+  $ chmod +x _bin/$bin
+
+  $ make_lockpkg <<EOF
+  > (version 1.0.0)
+  > (exported_env
+  >  (:= PATH $PWD/_bin))
+  > EOF
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.13)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (rule
+  >  (alias foo)
+  >  (action (run %{bin:$bin})))
+  > EOF
+
+  $ dune build @foo
+  File "dune", line 3, characters 14-30:
+  3 |  (action (run %{bin:foobarbin})))
+                    ^^^^^^^^^^^^^^^^
+  Error: Program foobarbin not found in the tree or in PATH
+   (context: default)
+  [1]
+
+  $ PATH=$PWD/_bin:$PATH $bin
+  foo