Verify that backend environment solving picks up the mirror config

tests/data/pixi_build/config-pickup-test/pixi.toml

@@ -0,0 +1,18 @@
+[workspace]
+channels = []
+platforms = ["win-64", "linux-64", "osx-arm64", "osx-64"]
+preview = ["pixi-build"]
+
+[dependencies]
+simple-config-test = { path = "." }
+
+[tasks]
+start = "simple-config-test"
+
+[package]
+name = "simple-config-test"
+version = "0.1.0"
+
+[package.build]
+backend = { name = "pixi-build-rattler-build", version = "*" }
+channels = ["https://broken.url/conda-forge"]

tests/data/pixi_build/config-pickup-test/recipe.yaml

@@ -0,0 +1,21 @@
+package:
+  name: simple-config-test
+  version: 0.1.0
+
+build:
+  number: 0
+  script:
+    - if: win
+      then:
+        - mkdir -p %PREFIX%\bin
+        - echo @echo off > %PREFIX%\bin\simple-config-test.bat
+        - echo echo Build backend works >> %PREFIX%\bin\simple-config-test.bat
+      else:
+        - mkdir -p $PREFIX/bin
+        - echo "#!/usr/bin/env bash" > $PREFIX/bin/simple-config-test
+        - echo "echo Build backend works" >> $PREFIX/bin/simple-config-test
+        - chmod +x $PREFIX/bin/simple-config-test
+
+requirements:
+  host:
+    - fd-find

tests/integration_python/test_config_pickup.py

@@ -0,0 +1,37 @@
+import shutil
+from pathlib import Path
+
+import tomli_w
+import tomllib
+
+from .common import verify_cli_command
+
+
+def test_config_pickup_by_build_backends(
+    pixi: Path, build_data: Path, tmp_pixi_workspace: Path
+) -> None:
+    """
+    Test that pixi build backends pick up config from .pixi/config.toml.
+
+    First tests with working config, then verifies the config is actually
+    being used by checking the log output for mirror usage.
+    """
+    # Copy our test workspace
+    test_data = build_data.joinpath("config-pickup-test")
+    shutil.copytree(test_data, tmp_pixi_workspace, dirs_exist_ok=True)
+
+    manifest_path = tmp_pixi_workspace.joinpath("pixi.toml")
+
+    # Create .pixi/config.toml with mirror that redirects from the broken URL to our backends channel
+    pixi_dir = tmp_pixi_workspace.joinpath(".pixi")
+    config_path = pixi_dir.joinpath("config.toml")
+    config = tomllib.loads(config_path.read_text())
+    config["mirrors"] = {
+        "https://broken.url/conda-forge": ["https://prefix.dev/conda-forge"],
+    }
+    config_path.write_text(tomli_w.dumps(config))
+
+    verify_cli_command(
+        [pixi, "run", "-v", "--manifest-path", manifest_path, "start"],
+        stdout_contains="Build backend works",
+    )