Remove BASH_BIN in favor of referencing bash via /usr/bin/env

While using rules_py in combination with rules_oci, I discovered an unexpected
behaviour: rules_py generates #! shebang lines using the path to bin that the
sh toolchain uses. This has the unexpected side effect of baking in the bash
path from the machine that built the container, which may or may not be the
same bash path that the container itself will have.

I discovered that macOS, Ubuntu, and the default @python docker image all have
bash at `/bin/bash`, but surprisingly, GitHub Actions workers have it at
`/usr/bin/bash`.

We can side step this problem entirely by using `#!/usr/bin/env bash` as our
shebang line, which I believe to be sufficiently portable and robust to use
for rules_py's scripts. See [this Stack Overflow](https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash)
question for further discussion on the approach.

Author: Mark Christian

MODULE.bazel.orig

@@ -0,0 +1,75 @@
+"aspect-build/rules_py"
+
+module(
+    name = "aspect_rules_py",
+    compatibility_level = 1,
+    version = "0.0.0",
+)
+
+# Lower-bound versions of direct dependencies.
+# When bumping, add a comment explaining what's required from the newer release.
+bazel_dep(name = "aspect_bazel_lib", version = "1.40.0")
+bazel_dep(name = "bazel_skylib", version = "1.4.2")
+bazel_dep(name = "rules_python", version = "0.29.0")
+bazel_dep(name = "platforms", version = "0.0.7")
+
+tools = use_extension("//py:extensions.bzl", "py_tools")
+tools.rules_py_tools()
+use_repo(tools, "rules_py_tools")
+
+register_toolchains(
+    "@rules_py_tools//:all",
+
+    # Register the "from source" toolchains last, so there's no accidental dependency on Rust
+    # For manual testing: comment these out to force use of pre-built binaries.
+    "@aspect_rules_py//py/private/toolchain/venv/...",
+    "@aspect_rules_py//py/private/toolchain/unpack/...",
+)
+
+# To allow Rust binaries in /py/tools to be built from source
+# NOTE: when publishing to BCR, we patch these to be dev_dependency, as we publish pre-built binaries
+# along with our releases.
+
+bazel_dep(
+    name = "rules_rust",
+    version = "0.38.0",
+    # In released versions: dev_dependency = True
+)
+
+rust = use_extension(
+    "@rules_rust//rust:extensions.bzl",
+    "rust",
+    # In released versions: dev_dependency = True
+)
+
+rust.toolchain(
+    edition = "2021",
+    versions = ["1.74.1"],
+)
+use_repo(rust, "rust_toolchains")
+
+register_toolchains(
+    "@rust_toolchains//:all",
+    # In released versions: dev_dependency = True
+)
+
+crate = use_extension(
+    "@rules_rust//crate_universe:extension.bzl",
+    "crate",
+    # In released versions: dev_dependency = True
+)
+
+crate.from_cargo(
+    name = "crate_index",
+    cargo_lockfile = "//:Cargo.lock",
+    # Apparently not needed under bzlmod?
+    # lockfile = "//:Cargo.Bazel.lock",
+    manifests = [
+        "//:Cargo.toml",
+        "//py/tools/py:Cargo.toml",
+        "//py/tools/venv_bin:Cargo.toml",
+        "//py/tools/unpack_bin:Cargo.toml",
+    ],
+)
+
+use_repo(crate, "crate_index")