Respect non-sysconfig-based system Pythons (#2193)

## Summary

`pip` uses `sysconfig` for Python 3.10 and later by default; however, it
falls back to `distutils` for earlier Python versions, and distros can
actually tell `pip` to continue falling back to `distutils` via the
`_PIP_USE_SYSCONFIG` variable.

By _always_ using `sysconfig`, we're doing the wrong then when
installing into some system Pythons, e.g., on Debian prior to Python
3.10.

This PR modifies our logic to mirror `pip` exactly, which is what's been
recommended to me as the right thing to do.

Closes https://github.com/astral-sh/uv/issues/2113.

## Test Plan

Most notably, the new Debian tests pass here (which fail on main:
https://github.com/astral-sh/uv/pull/2144).

I also added Pyston as a second stress-test.

.github/workflows/system-install.yml

@@ -18,6 +18,30 @@ env:
   RUSTUP_MAX_RETRIES: 10
 
 jobs:
+  install-debian:
+    name: "Install Python on Debian"
+    runs-on: ubuntu-latest
+    container: debian:bullseye
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Install Python"
+        run: apt-get update && apt-get install -y python3.9 python3-pip python3.9-venv
+
+      - name: "Install Rust toolchain"
+        run: apt-get update && apt-get install -y curl build-essential && curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: $HOME/.cargo/bin/cargo build --no-default-features
+
+      - name: "Print Python path"
+        run: echo $(which python3.9)
+
+      - name: "Validate global Python install"
+        run: python3.9 scripts/check_system_python.py --uv ./target/debug/uv
+
   install-ubuntu:
     name: "Install Python on Ubuntu"
     runs-on: ubuntu-latest
@@ -66,6 +90,27 @@ jobs:
       - name: "Validate global Python install"
         run: pypy scripts/check_system_python.py --uv ./target/debug/uv
 
+  install-pyston:
+    name: "Install Pyston"
+    runs-on: ubuntu-latest
+    container: pyston/pyston:2.3.5
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Install Rust toolchain"
+        run: apt-get update && apt-get install -y curl build-essential && curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: $HOME/.cargo/bin/cargo build --no-default-features
+
+      - name: "Print Python path"
+        run: echo $(which pyston)
+
+      - name: "Validate global Python install"
+        run: pyston scripts/check_system_python.py --uv ./target/debug/uv
+
   install-macos:
     name: "Install Python on macOS"
     runs-on: macos-14