Fix GitHub CI flow to accommodate removed configure script

Commit b0aa9df ("Remove configure script") as it says, removed
"configure" script and since then the VI was failing. Fix it by autogeneration
of the missing script before its execution as suggested by the offending commit.

Author: abrodkin

.github/workflows/build-toolchains.yml

@@ -100,6 +100,8 @@ jobs:
 
           echo ::set-output name=toolchain_name::${{ matrix.target }}-${MODE}-${RELEASE_TAG}
 
+          autoconf
+
           ${{ github.workspace }}/configure \
             ${BUILD_FLAGS} \
             --target=${{ matrix.target }} \