Move Chipyard CI to Github Actions

- As similar as possible to the circle ci code.
- The `.github/README.md` file has a fair amount of documentation for this.
- Creates a worfklow file
  - re-uses most of the circleci/scipts unchanged
  - defines a number of *Composite Actions* which are like YML subroutines
- Removes the circle-ci code
- Points the CI badge in the top level README to use the GA test result

.github/scripts/build-extra-tests.sh

@@ -7,9 +7,5 @@ set -ex
 SCRIPT_DIR="$( cd "$( dirname "$0" )" && pwd )"
 source $SCRIPT_DIR/defaults.sh
 
-export RISCV="$GITHUB_WORKSPACE/riscv-tools-install"
-export LD_LIBRARY_PATH="$RISCV/lib"
-export PATH="$RISCV/bin:$PATH"
-
 make -C $LOCAL_CHIPYARD_DIR/tests clean
 make -C $LOCAL_CHIPYARD_DIR/tests