[native] Double quote BUILD_ARCH in build_openssl.sh

Summary: Quick double quote added that I missed in D4716.

Test Plan:
ShellCheck.

Additionally, build_openssl.sh is run during the Android build so if that workflow continues to pass CI we can reasonably assume that the script continues to work as expected.

Reviewers: atul, jon, tomek

Reviewed By: jon

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4737