Rust 1.20 fails a test on core-updates, possibly because of the new
version of GNU Make (4.3).

I suppose we can disable that test for the bootstrap builds as long as
it works for the latest version of Rust.

Log output:

     Running build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/deps/jobserver-dd0c6bf78d70e32b

running 3 tests
test jobserver_and_j ... ok
test jobserver_exists ... ok
test makes_jobserver_used ... FAILED

failures:

---- makes_jobserver_used stdout ----
        running `make -j2`
thread 'makes_jobserver_used' panicked at '
Expected: execs
    but: exited with exit code: 2
--- stdout
/tmp/guix-build-rust-1.20.0.drv-0/rustc-1.20.0-src/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/cargo build

--- stderr
   Compiling d1 v0.0.1 (file:///tmp/guix-build-rust-1.20.0.drv-0/rustc-1.20.0-src/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/cit/t2/foo/d1)
   Compiling d2 v0.0.1 (file:///tmp/guix-build-rust-1.20.0.drv-0/rustc-1.20.0-src/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/cit/t2/foo/d2)
error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.20.0 running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'failed to acquire jobserver token: Error { repr: Os { code: 11, message: "Resource temporarily unavailable" } }', src/libcore/result.rs:860:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.20.0 running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'failed to acquire jobserver token: Error { repr: Os { code: 11, message: "Resource temporarily unavailable" } }', src/libcore/result.rs:860:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: failed to acquire jobserver token

Caused by:
  Resource temporarily unavailable (os error 11)
make: *** [Makefile:2: all] Error 101
', src/vendor/hamcrest/src/core.rs:31:12
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    makes_jobserver_used

test result: FAILED. 2 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--test jobserver'


command did not execute successfully: "/gnu/store/c3jc4rlyj1b50djxny0ldpbpywaf5apr-rust-1.19.0-cargo/bin/cargo" "test" "-j" "1" "--target" "x86_64-unknown-linux-gnu" "--release" "--frozen" "--manifest-path" "/tmp/guix-build-rust-1.20.0.drv-0/rustc-1.20.0-src/src/tools/cargo/Cargo.toml"
expected success, got: exit code: 101


failed to run: /tmp/guix-build-rust-1.20.0.drv-0/rustc-1.20.0-src/build/bootstrap/debug/bootstrap -j1 test src/tools/cargo
Build completed unsuccessfully in 0:09:46
command "./x.py" "-j1" "test" "src/tools/cargo" failed with status 1
builder for `/gnu/store/dxrkx2iqr0vhan7m0lfczw3b8rpyw8z8-rust-1.20.0.drv' failed with exit code 1