CI: fix MacOS jobs

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     name: Build and Test
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-13]
         scala: [2.13.13]
         java:
           - temurin@8

build.sbt

@@ -15,7 +15,16 @@ inThisBuild(Seq[Setting[_]](
     Developer("retronym", "Jason Zaugg", "@retronym", url("https://github.com/retronym")),
   ),
   scalacOptions := Seq("-feature", "-deprecation", "-Xlint", "-Werror"),
-  githubWorkflowOSes := Seq("ubuntu-latest", "macos-latest"),
+  // since April 2024 or so, macos-latest gives us a machine that doesn't
+  // have sbt installed (actions/setup-java#627). if we weren't using
+  // sbt-github-actions it would be easy to just edit the workflow directly to
+  // use `brew install sbt`, but it's easier to just stay on macos-13 for as
+  // long as it remains available. I've reported the issue upstream at
+  // sbt/sbt-github-actions#185 . if we do move
+  // to macos-latest, note that it doesn't have temurin@8 anymore (as per
+  // actions/setup-java#625), so we'd also need to change that entry (a few
+  // lines below) from "temurin" to "zulu"
+  githubWorkflowOSes := Seq("ubuntu-latest", "macos-13"),
   githubWorkflowJavaVersions := Seq(
     JavaSpec.temurin("8"),
     JavaSpec.temurin("11"),