fix: change the way to decide whether there's a RUSTC_WORKSPACE_WRAPPER

[BugenZhao]

#1280 added support for locating the rustc binary when there's a workspace wrapper set via RUSTC_WORKSPACE_WRAPPER. A check on the existence of env var CARGO was included as a guardrail against false positives.

However, this doesn't seem to always work. In my reproduction at https://github.com/bugenzhao/sccache-wrapper-reproduce, it seems that the environment variable CARGO is not set when cargo attempts to detect the compiler as its initial step:

https://github.com/rust-lang/cargo/blob/2251525ae503fa196f6d7f9ce6d32eccb2d5f044/src/cargo/util/rustc.rs#L62-L66

As a result, we fail to decide the maybe_rustc_executable and eventually run into the testing of C compiler:

sccache: caused by: Compiler not supported: "/risingwave/ci/scripts/rustc-workspace-wrapper.sh: line 16: exec: -E: invalid option\nexec: usage: exec [-cl] [-a name] [command [argument ...]] [redirection ...]\n"

This PR attempts to fix this by directly checking whether RUSTC_WORKSPACE_WRAPPER is set.

[sylvestre]

please add tests to make sure we don't regress in the future
maybe in
https://github.com/mozilla/sccache/blob/main/tests/sccache_cargo.rs
or
https://github.com/mozilla/sccache/blob/main/tests/sccache_rustc.rs

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.18%. Comparing base (7025295) to head (bb6ff20).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2408      +/-   ##
==========================================
+ Coverage   67.06%   67.18%   +0.11%     
==========================================
  Files          64       64              
  Lines       34718    34720       +2     
==========================================
+ Hits        23285    23327      +42     
+ Misses      11433    11393      -40     

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