Fix buildkite static-stack action.

The glob substitution was run _before_ the script, so that
the `cd static-stack` was not applied and thus the path was invalid.

Author: nh2

.buildkite/pipeline.yml

@@ -48,12 +48,4 @@ steps:
   # Stack via stack2nix
 
   - label: static-stack
-    # TODO: Remove the override of unix-compat below once that's available in stack
-    command: |
-      set -eu -o pipefail
-
-      cd static-stack/
-      mkdir -p static-stack-test-dir
-      curl -L https://github.com/commercialhaskell/stack/archive/v2.7.1.tar.gz | tar -xz -C static-stack-test-dir
-
-      $(nix-build --no-link -A fullBuildScript --argstr stackDir $PWD/static-stack-test-dir/stack-*)
+    command: "./static-stack/build-static-stack.sh"

static-stack/build-static-stack.sh

@@ -0,0 +1,8 @@
+#! /usr/bin/env bash
+
+set -eu -o pipefail
+
+mkdir -p static-stack-test-dir
+curl -L https://github.com/commercialhaskell/stack/archive/v2.7.1.tar.gz | tar -xz -C static-stack-test-dir
+
+$(nix-build --no-link -A fullBuildScript --argstr stackDir $PWD/static-stack-test-dir/stack-*)