ci: build i686-unknown-linux-gnu distributions

We just implemented support for 32-bit cross-compiling. Let's get it
hooked up to CI.

Author: indygreg

.github/workflows/linux.yml

@@ -122,6 +122,7 @@ jobs:
       fail-fast: false
       matrix:
         triple:
+          - 'i686-unknown-linux-gnu'
           - 'x86_64-unknown-linux-gnu'
           - 'x86_64-unknown-linux-musl'
         py:
@@ -136,6 +137,7 @@ jobs:
           - 'pgo+lto'
         exclude:
           # We don't publish noopt builds for GNU Linux.
+          - triple: 'i686-unknown-linux-gnu'
           - triple: 'x86_64-unknown-linux-gnu'
             optimizations: 'noopt'
           # PGO doesn't work with musl libc.