fix(maker-squirrel): use executableName for exe when available

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.

Summarize your changes:

Fixes a bug reported in Discord.

Steps to reproduce:

npx create-electron-app example-app
cd example-app

Update package.json and merge the following instructions in:

{
  "productName": "My Example App",
  "config": {
    "forge": {
      "packagerConfig": {
        "executableName": "MyExampleApp"
      }
    }
  }
}

Expected: Squirrel installer finds MyExampleApp.exe

Actual: Squirrel installer tries and fails to look for My Example App.exe.

Solution: fix the default exe option to check forgeConfig.packagerConfig.executableName first.

[codecov]

Codecov Report

Merging #2365 (09c8f72) into master (7aa22e6) will decrease coverage by 2.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2365      +/-   ##
==========================================
- Coverage   75.56%   73.16%   -2.40%     
==========================================
  Files          74       74              
  Lines        2202     2199       -3     
  Branches      415      416       +1     
==========================================
- Hits         1664     1609      -55     
+ Misses        421      418       -3     
- Partials      117      172      +55     

Impacted Files	Coverage Δ
packages/maker/squirrel/src/MakerSquirrel.ts	78.26% <100.00%> (+0.98%)	⬆️
packages/api/core/src/api/publish.ts	69.73% <0.00%> (-22.37%)	⬇️
packages/api/core/src/api/start.ts	67.08% <0.00%> (-20.26%)	⬇️
packages/api/core/src/api/install.ts	78.20% <0.00%> (-16.67%)	⬇️
packages/api/core/src/util/rebuild.ts	92.85% <0.00%> (-7.15%)	⬇️
packages/api/core/src/api/make.ts	81.72% <0.00%> (-6.58%)	⬇️
packages/api/core/src/api/init.ts	91.17% <0.00%> (-5.89%)	⬇️
packages/api/core/src/util/upgrade-forge-config.ts	96.42% <0.00%> (+1.69%)	⬆️
packages/installer/darwin/src/InstallerDarwin.ts	33.33% <0.00%> (+1.75%)	⬆️
packages/maker/base/src/Maker.ts	77.77% <0.00%> (+2.10%)	⬆️

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