Add support for aarch64 (#102)

Setting the `arch` input to `aarch64` will install `aarch64` binaries,
intended to be used on aarch64-based self-hosted runners.

Please note that we currently don't run test builds on that
architecture, so we cannot guarantee that the input won't break
randomly, although there is no reason why it would.

In a future update, we may choose a default architecture based on the
arch of the runner.

Thanks to @giordano for providing a self-hosted runner for testing.

Co-authored-by: Mosè Giordano <mose@gnu.org>

src/installer.ts

@@ -18,7 +18,8 @@ const osMap = {
 }
 const archMap = {
     'x86': 'i686',
-    'x64': 'x86_64'
+    'x64': 'x86_64',
+    'aarch64': 'aarch64'
 }
 
 // Store information about the environment