Add a trampoline variant that just executes python (#8637)

Currently, our trampoline is used to convert `<command> [args]` to
`python <command> [args]` for script entrypoints installed into virtual
environments. For #8458, it'd be nice to convert a shim `python3.12
[args]` to `python [args]`. Here, we modify the trampolines to support
this use-case.

The only change we really need here is to avoid injecting `<command>`
into the child process. We change the "magic number" at the end of the
trampoline executables from `UVUV` to `UVSC` and `UVPY` which define
"script" and "python" variants to the trampoline. We then omit the
`<command>` injection in the latter case. We also omit writing the zip
script payload.

To support construction of the new variant, a new
`uv-trampoline-builder` crate is introduced — this avoids requirements
on `uv-install-wheel` in future work. I also use `uv-trampoline-builder`
to consolidate some of the test setup for `uv-trampoline`.

There should be no backwards compatibility concerns, since trampolines
are fully self-referential.

I rebased to fix the commits at the end, as this took many iterations to
get working via CI. This should roughly be reviewable by commit if you
prefer.

.github/workflows/ci.yml

@@ -396,6 +396,11 @@ jobs:
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: ${{ github.workspace }}/crates/uv-trampoline
+      - name: "Test committed binaries"
+        working-directory: ${{ github.workspace }}
+        run: |
+          rustup target add ${{ matrix.target-arch }}-pc-windows-msvc
+          cargo test -p uv-trampoline-builder --target ${{ matrix.target-arch }}-pc-windows-msvc
       # Build and copy the new binaries
       - name: "Build"
         working-directory: ${{ github.workspace }}/crates/uv-trampoline
@@ -403,9 +408,11 @@ jobs:
           cargo build --target ${{ matrix.target-arch }}-pc-windows-msvc
           cp target/${{ matrix.target-arch }}-pc-windows-msvc/debug/uv-trampoline-console.exe trampolines/uv-trampoline-${{ matrix.target-arch }}-console.exe
           cp target/${{ matrix.target-arch }}-pc-windows-msvc/debug/uv-trampoline-gui.exe trampolines/uv-trampoline-${{ matrix.target-arch }}-gui.exe
-      - name: "Test"
-        working-directory: ${{ github.workspace }}/crates/uv-trampoline
-        run: cargo test --target ${{ matrix.target-arch }}-pc-windows-msvc --test *
+      - name: "Test new binaries"
+        working-directory: ${{ github.workspace }}
+        run: |
+          # We turn off the default "production" test feature since these are debug binaries
+          cargo test -p uv-trampoline-builder --target ${{ matrix.target-arch }}-pc-windows-msvc --no-default-features
 
   typos:
     runs-on: ubuntu-latest