Eliminate xargo dependency

[neuschaefer]

xargo is no longer actively developed and has fortunately become unnecessary due to the build-std option in cargo.

Fixes: #692

---

The plan is to try all targets and see which ones actually need build-std. I will then add build-std in targets.toml as needed. It is also possible that I'm missing something else.

[a5ehren]

Not sure when it quit working, but note that at least as of Rust 1.91 you can no longer build an image from the Dockerfiles due to Xargo:

25.83    Compiling dirs-sys v0.3.7
25.83    Compiling fs2 v0.4.3
25.91    Compiling dirs v4.0.0
26.23    Compiling tempdir v0.3.7
27.15    Compiling toml v0.5.11
28.00 error[E0308]: mismatched types
28.00    --> /tmp/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/xargo-0.3.26/src/flock.rs:118:41
28.00     |
28.00 118 |                 acquire(msg, &path, &|| f.try_lock_shared(), &|| f.lock_shared())?;
28.00     |                                         ^^^^^^^^^^^^^^^^^^^ expected `Result<(), Error>`, found `Result<(), TryLockError>`
28.00     |
28.00     = note: expected enum `std::result::Result<_, std::io::Error>`
28.00                found enum `std::result::Result<_, std::fs::TryLockError>`
28.00 help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
28.00     |
28.00 118 |                 acquire(msg, &path, &|| Ok(f.try_lock_shared()?), &|| f.lock_shared())?;
28.00     |                                         +++                   ++
28.00
28.12 For more information about this error, try `rustc --explain E0308`.
28.12 error: could not compile `xargo` (lib) due to 1 previous error
28.12 warning: build failed, waiting for other jobs to finish...
28.80 error: failed to compile `xargo v0.3.26`, intermediate artifacts can be found at `/tmp/cargo-installH3IbjE`.
28.80 To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.
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Dockerfile.x86_64-pc-windows-gnu:11
--------------------
   9 |
  10 |     COPY xargo.sh /
  11 | >>> RUN /xargo.sh
  12 |
  13 |     FROM cross-base AS build
--------------------
ERROR: failed to build: failed to solve: process "/bin/sh -c /xargo.sh" did not complete successfully: exit code: 101