After checking your PKGBUILD, I've found three problems. You don't need to export RUSTUP_TOOLCHAIN=stable more than once for it work you can do it once and it will remain stable. Secondly you don't need to include the license verbatim in the PKGBUILD's repository, if you wish to install the license you can get it from upstream's Git repository instead. Also the arch array says it can be built on i686,x86_64,armv6h,armv7h; given that it's for rmpc which only supports aarch64 and x84_64 it would be better to say those are the supported architectures for this package.
Keon Cachia
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