Update Linux platform to specify GNU and Musl versions

In Nokogiri 18, the gem started shipping separate precompiled GNU and
Musl gems for all Linux platforms.  Previously both GNU and Musl target
systems could use and install the same gem, e.g., the platform gem for
x86_64-linux (see [Nokogiri 18 release notes](https://github.com/sparklemotion/nokogiri/releases/tag/v1.18.0) for details).

This broke our Docker build. To fix it, this commit specifies that we
want to use both versions of the gem on Linux (since we use Alpine Linux
in our Docker images and Musl is the default C library in Alpine, but
other places like our Github Actions use the GNU version).

Gemfile.lock

@@ -352,12 +352,14 @@ GEM
     zeitwerk (2.7.1)
 
 PLATFORMS
-  aarch64-linux
+  aarch64-linux-gnu
+  aarch64-linux-musl
   arm64-darwin-22
   arm64-darwin-23
   x86_64-darwin-22
   x86_64-darwin-23
-  x86_64-linux
+  x86_64-linux-gnu
+  x86_64-linux-musl
 
 DEPENDENCIES
   aws-sdk-codepipeline (~> 1.93)