Add a --system flag for opt-in non-virtualenv installs (#2046)

## Summary

This is essentially a wrapper around something like `--python $(which
python3)`, but gives users a portable and streamlined way to solve the
common pain point of using `uv` in GitHub Actions or a Docker container.

See: https://github.com/astral-sh/uv/issues/1526.

.github/workflows/system-install.yml

@@ -0,0 +1,110 @@
+name: System Install
+
+on:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+env:
+  CARGO_INCREMENTAL: 0
+  CARGO_NET_RETRY: 10
+  CARGO_TERM_COLOR: always
+  RUSTUP_MAX_RETRIES: 10
+
+jobs:
+  install-ubuntu:
+    name: "Install Python on Ubuntu"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: "Install Rust toolchain"
+        run: rustup show
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: cargo build
+
+      - name: "Print Python path"
+        run: echo $(which python)
+
+      - name: "Validate global Python install"
+        run: python scripts/check_system_python.py --uv ./target/debug/uv
+
+  install-macos:
+    name: "Install Python on macOS"
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Install Python"
+        run: brew install python@3.8
+
+      - name: "Install Rust toolchain"
+        run: rustup show
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: cargo build
+
+      - name: "Print Python path"
+        run: echo $(which python3.11)
+
+      - name: "Validate global Python install"
+        run: python3.11 scripts/check_system_python.py --uv ./target/debug/uv
+
+  install-windows:
+    name: "Install Python on Windows"
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+
+#      - name: "Install Python"
+#        run: choco install python
+
+      - name: "Install Rust toolchain"
+        run: rustup show
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: cargo build
+
+      - name: "Print Python path"
+        run: echo $(which python)
+
+      - name: "Validate global Python install"
+        run: py -3.12 ./scripts/check_system_python.py --uv ./target/debug/uv
+
+  install-pyenv:
+    name: "Install Python using pyenv"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Install pyenv"
+        uses: "gabrielfalcao/pyenv-action@v18"
+        with:
+          default: 3.9.7
+
+      - name: "Install Rust toolchain"
+        run: rustup show
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: "Build"
+        run: cargo build
+
+      - 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