Refactor signing certificate handling in "Arduino IDE" workflow

Previously, there was some code duplication of the complex code signing certificate handling commands, which made the
related code more difficult to understand, maintain, and develop.

The cause of this duplication is that there is a separate certificate for each operating system, each of which is stored
in separate repository secrets, as well as a different certificate file extension for each OS. Since the secret names
and file extensions are associated with the operating system, it is most logical to define them via attributes alongside
the operating system definition in the job matrix configuration already used to generate the parallel job runs for
native build on each OS.

That done, the certificate handling commands are universal and the system can easily expand to additional host targets
(e.g., Apple M1) as time goes on.

Author: per1234

.github/workflows/build.yml

@@ -17,13 +17,22 @@ env:
 
 jobs:
   build:
+    name: build (${{ matrix.config.os }})
     if: github.repository == 'arduino/arduino-ide'
     strategy:
       matrix:
         config:
           - os: 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-18.04 # https://github.com/arduino/arduino-ide/issues/259
           - os: 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 }}
     timeout-minutes: 90
 
@@ -59,23 +68,14 @@ jobs:
           if [ $IS_FORK = true ]; then
             echo "Skipping the app signing: building from a fork."
           else
-            if [ "${{ runner.OS }}" = "macOS" ]; then
-              export CSC_LINK="${{ runner.temp }}/signing_certificate.p12"
-              # APPLE_SIGNING_CERTIFICATE_P12 secret was produced by following the procedure from:
-              # https://www.kencochrane.com/2020/08/01/build-and-sign-golang-binaries-for-macos-with-github-actions/#exporting-the-developer-certificate
-              echo "${{ secrets.APPLE_SIGNING_CERTIFICATE_P12 }}" | base64 --decode > "$CSC_LINK"
-
-              export CSC_KEY_PASSWORD="${{ secrets.KEYCHAIN_PASSWORD }}"
-
-            elif [ "${{ runner.OS }}" = "Windows" ]; then
-              export CSC_LINK="${{ runner.temp }}/signing_certificate.pfx"
-              npm config set msvs_version 2017 --global
-              echo "${{ secrets.WINDOWS_SIGNING_CERTIFICATE_PFX }}" | base64 --decode > "$CSC_LINK"
-
-              export CSC_KEY_PASSWORD="${{ secrets.WINDOWS_SIGNING_CERTIFICATE_PASSWORD }}"
-            fi
+            export CSC_LINK="${{ runner.temp }}/signing_certificate.${{ matrix.config.certificate-extension }}"
+            echo "${{ secrets[matrix.config.certificate-secret] }}" | base64 --decode > "$CSC_LINK"
+            export CSC_KEY_PASSWORD="${{ secrets[matrix.config.certificate-password-secret] }}"
           fi
 
+          if [ "${{ runner.OS }}" = "Windows" ]; then
+            npm config set msvs_version 2017 --global
+          fi
           npx node-gyp install
           yarn --cwd ./electron/packager/
           yarn --cwd ./electron/packager/ package