Use separate attributes for human identifier and runner identifier in build job matrix

The "Arduino IDE" GitHub Actions workflow uses a job matrix to make the builds for each host target in parallel.

The same steps are used for each job, but some configuration adjustments must be made on a per-target basis. This is
done through various attributes in the matrix configuration.

Previously the `os` attribute was used for two distinct things:

- The machine identifier of the GitHub Actions runner machine of the job.
- The differentiator in the human-targeted job name.

The attribute name "os" (for "operating system") was misleading because runners are differentiated by more than only the
operating system on the machine.

The use of a machine identifier as a differentiator in the human-targeted job name was a bad idea because these
identifiers would only effectively communicate the nature of a build to humans who are quite knowledgeable about the
GitHub Actions workflow syntax.

The impact of these poor decisions has not been too severe previously due to there only being a single job for each
operating system. However, there is a need for multiple jobs per operating system in order to support multiple host
architectures (e.g., macOS x86 and ARM).

The solution is to:

- Use an appropriate name for the runner identifier attribute.
- Use a dedicated attribute for the human friendly job name differentiator.

.github/workflows/build.yml

@@ -61,24 +61,27 @@ jobs:
           echo "result=$RESULT" >> $GITHUB_OUTPUT
 
   build:
-    name: build (${{ matrix.config.os }})
+    name: build (${{ matrix.config.name }})
     needs: run-determination
     if: needs.run-determination.outputs.result == 'true'
     strategy:
       matrix:
         config:
-          - os: windows-2019
+          - name: Windows # Human identifier for the job.
+            runs-on: windows-2019
             certificate-secret: WINDOWS_SIGNING_CERTIFICATE_PFX # Name of the secret that contains the certificate.
             certificate-password-secret: WINDOWS_SIGNING_CERTIFICATE_PASSWORD # Name of the secret that contains the certificate password.
             certificate-extension: pfx # File extension for the certificate.
-          - os: ubuntu-20.04
-          - os: macos-latest
+          - name: Linux
+            runs-on: ubuntu-20.04
+          - name: macOS x86
+            runs-on: macos-latest
             # APPLE_SIGNING_CERTIFICATE_P12 secret was produced by following the procedure from:
             # https://www.kencochrane.com/2020/08/01/build-and-sign-golang-binaries-for-macos-with-github-actions/#exporting-the-developer-certificate
             certificate-secret: APPLE_SIGNING_CERTIFICATE_P12
             certificate-password-secret: KEYCHAIN_PASSWORD
             certificate-extension: p12
-    runs-on: ${{ matrix.config.os }}
+    runs-on: ${{ matrix.config.runs-on }}
     timeout-minutes: 90
 
     steps: