Fix tsan tests.

from [this](https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html#threadsanitizer)

> To work correctly ThreadSanitizer needs to be "aware" of all synchronization operations in a program. It generally achieves that through \[...\] and compile time instrumentation (e.g. atomic operations). Using it without instrumenting all the program code can lead to false positive reports.

and the example uses -Zbuild-std ... of course, std has to be instrumented. Adding -Zbuild-std fixes it, indeed.

.github/workflows/build.yml

@@ -295,7 +295,7 @@ jobs:
         with:
           use-cross: false
           command: test
-          args: --test tsan --target=${{ matrix.target }} --features=${{ matrix.features }} ${{ matrix.buildtype }} -- --test-threads=1
+          args: -Zbuild-std --test tsan --target=${{ matrix.target }} --features=${{ matrix.features }} ${{ matrix.buildtype }} -- --test-threads=1
 
   # Run cfail tests on MSRV
   testcfail: