ci: reduce the number of tests in CI (#540)

Seems like we have a few too many tests that aren't really needed for just asserting CI is passing.

The goal for CI (IMO) is just to ensure things still build on the various supported platforms after changes are made.  However, there were a few tested scenarios like Windows GNU or musl which I feel weren't really too important in this regard, and added extra time to an already long CI process.

Commented out the following tests since there aren't any architecture-specific features that require running these in addition to other already-existing tests:
- Windows GNU
- Linux musl (both x86 and x86_64)

Of course, should we add changes that directly affect these architectures, then we should add the tests back.

.github/workflows/ci.yml

@@ -91,18 +91,18 @@ jobs:
               cross: true,
               rust: stable,
             }
-          - {
-              os: "ubuntu-latest",
-              target: "x86_64-unknown-linux-musl",
-              cross: false,
-              rust: stable,
-            }
-          - {
-              os: "ubuntu-latest",
-              target: "i686-unknown-linux-musl",
-              cross: true,
-              rust: stable,
-            }
+          # - {
+          #     os: "ubuntu-latest",
+          #     target: "x86_64-unknown-linux-musl",
+          #     cross: false,
+          #     rust: stable,
+          #   }
+          # - {
+          #     os: "ubuntu-latest",
+          #     target: "i686-unknown-linux-musl",
+          #     cross: true,
+          #     rust: stable,
+          #   }
           - {
               os: "macOS-latest",
               target: "x86_64-apple-darwin",
@@ -117,12 +117,6 @@ jobs:
           #     cross: false,
           #     rust: stable,
           #   }
-          - {
-              os: "windows-2019",
-              target: "i686-pc-windows-msvc",
-              cross: true,
-              rust: stable,
-            }
           - {
               os: "windows-2019",
               target: "x86_64-pc-windows-msvc",
@@ -132,10 +126,16 @@ jobs:
             }
           - {
               os: "windows-2019",
-              target: "x86_64-pc-windows-gnu",
-              cross: false,
+              target: "i686-pc-windows-msvc",
+              cross: true,
               rust: stable,
             }
+          # - {
+          #     os: "windows-2019",
+          #     target: "x86_64-pc-windows-gnu",
+          #     cross: false,
+          #     rust: stable,
+          #   }
 
           # aarch64
           - {