Allow installation of manylinux wheels on riscv64

auditwheel is capable of generating riscv64 wheels for manylinux_2_31
and above.  Here we modify uv-platform-tags so that those wheels can
be installed using uv.

Fixes: #8889

crates/uv-platform-tags/src/platform.rs

@@ -117,8 +117,10 @@ impl Arch {
             }
             // manylinux 1
             Self::X86 | Self::X86_64 => Some(5),
+            // manylinux_2_31
+            Self::Riscv64 => Some(31),
             // unsupported
-            Self::Armv6L | Self::Riscv64 => None,
+            Self::Armv6L => None,
         }
     }
 }