Better error message when git is not found

[blueraft]

Summary

Closes #9200

Test Plan

Using the following Dockerfile:

FROM debian:latest

RUN apt-get update && apt-get install -y python3

WORKDIR /app
COPY target/debug/uv .
RUN chmod +x uv

RUN /app/uv venv && /app/uv pip install [email protected]:pallets/flask.git

❯ cargo build -q -p uv && docker build .
...
 => ERROR [6/6] RUN /app/uv venv && /app/uv pip install [email protected]:pallets/flask.git                    0.4s
------
 > [6/6] RUN /app/uv venv && /app/uv pip install [email protected]:pallets/flask.git:
0.275 Using CPython 3.11.2 interpreter at: /usr/bin/python3
0.275 Creating virtual environment at: .venv
0.318   × Failed to download and build `git @
0.318   │ file:///app/github.com:pallets/flask.git`
0.318   ├─▶ Git operation failed
0.318   ╰─▶ Git executable not found. Ensure that Git is installed and available.
------
Dockerfile:9
--------------------
   7 |     RUN chmod +x uv
   8 |
   9 | >>> RUN /app/uv venv && /app/uv pip install [email protected]:pallets/flask.git
  10 |
--------------------
ERROR: failed to solve: process "/bin/sh -c /

[charliermarsh]

I think we should make this #[error(transparent)] and pass the source unmodified.

[blueraft]

Done