run xtest bash shell

For some reason cargo-xtest cannot be found on Windows if the shell
is not set to bash. This used to work and regressed about two weeks ago
for not immediately obvious reason. Considering that there is no big
downside to running xtest with the bash shell, we'll just use that for
now.

See https://github.com/rust-osdev/x86_64/actions/runs/4350740158 as an
example of a run that fails because cargo-xtest cannot be found.

Author: Freax13

.github/workflows/build.yml

@@ -186,6 +186,7 @@ jobs:
 
       - name: "Run Test Framework"
         run: cargo xtest
+        shell: bash
         working-directory: "testing"
 
   check_formatting: