Add riscv64 architecture support to platform detection (#791)

Add `riscv64gc` as a recognized architecture in the `Architecture` type
union and map Node.js's `riscv64` process.arch value to the `riscv64gc`
uv platform identifier.

This allows the action to correctly detect and download the appropriate
uv binary on RISC-V 64-bit systems (e.g., linux/riscv64 runners).

dist/save-cache/index.js

@@ -91307,6 +91307,7 @@ function getArch() {
         arm64: "aarch64",
         ia32: "i686",
         ppc64: "powerpc64le",
+        riscv64: "riscv64gc",
         s390x: "s390x",
         x64: "x86_64",
     };