Rust version compatibility

[eitsupi]

Both R-hub builder and R-universe builder (ubuntu:jammy) failed installation with the following error after using prql-compiler 0.4.0.
https://github.com/r-universe/eitsupi/actions/runs/3941624440/jobs/6752975644#step:3:802

  error[E0658]: `let...else` statements are unstable
     --> /tmp/RtmpnXSnJF/R.INSTALLfa93bec8912/prqlr/src/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/prql-compiler-0.4.0/src/semantic/lowering.rs:453:13
      |
  453 | /             let pl::ExprKind::All { except, .. } = expr.kind else {
  454 | |                 // base case
  455 | |                 r.push(self.declare_as_column(expr, is_aggregation)?);
  456 | |                 continue;
  457 | |             };
      | |______________^
      |
      = note: see issue #87335 <https://github.com/rust-lang/rust/issues/87335> for more information

[eitsupi]

#38 does not seem enough for R-universe.

   	cargo build --lib --release --manifest-path=./rust/Cargo.toml --target-dir ./rust/target
      Updating git repository `https://github.com/extendr/extendr`
      Updating crates.io index
      Updating git repository `https://github.com/PRQL/prql`
  error: failed to get `prql-compiler` as a dependency of package `prqlr v0.1.0 (/tmp/RtmpilNWCq/Rbuild5ef55c3be2a/prqlr/src/rust)`
  
  Caused by:
    failed to load source for dependency `prql-compiler`
  
  Caused by:
    Unable to update https://github.com/PRQL/prql?rev=1b4217c01c590c51a3e80c8a11b28f78f109a110#1b4217c0
  
  Caused by:
    object not found - no match for id (1b4217c01c590c51a3e80c8a11b28f78f109a110); class=Odb (9); code=NotFound (-3)
  make: *** [Makevars:16: rust/target/release/libprqlr.a] Error 101
  ERROR: compilation failed for package ‘prqlr’

[eitsupi]

Curiously, the following build (ubuntu:jammy based rocker/r-ver) succeeds.

-> The installation was failing, but it just wasn't getting build errors......

docker build -t prqlr-test -<<EOF
FROM rocker/r-ver
RUN apt-get update && apt-get install -y --no-install-recommends cargo git
RUN R -s -e 'install.packages("remotes")'
RUN R -q -e 'remotes::install_git("https://github.com/eitsupi/prqlr.git", ref = "9131d1cc5e227f368e57015aa29819c34db15a19")'
EOF

[eitsupi]

The cause was the use of the original URL even though it is ref of fork commit, and the use of the version.workspace.

[eitsupi]

Reopened by #40...