Fix release workflow: output executable to build/ subdirectory to avoid directory conflict

[Copilot]

raco exe -o randstr fails because randstr is an existing source directory, causing delete-file: cannot delete file - Is a directory.

Changes

• Output executable to build/ subdirectory instead of repository root
• Split build step into Unix/Windows variants for proper shell compatibility
  • Unix: mkdir -p build && raco exe -o build/randstr
  • Windows: New-Item -ItemType Directory -Force -Path build + raco exe -o build/randstr.exe
• Updated raco distribute commands to reference new paths

Original prompt

Run raco exe -o randstr randstr/cli/main.rkt
delete-file: cannot delete file
path: /home/runner/work/randstr/randstr/randstr
system error: Is a directory; errno=21
context...:
/usr/share/racket/collects/compiler/embed.rkt:1499:0: create-embedding-executable
/usr/share/racket/pkgs/compiler-lib/compiler/commands/exe.rkt:100:0
body of "/usr/share/racket/pkgs/compiler-lib/compiler/commands/exe.rkt"
/usr/share/racket/collects/raco/raco.rkt:41:0
body of "/usr/share/racket/collects/raco/raco.rkt"
body of "/usr/share/racket/collects/raco/main.rkt"
Error: Process completed with exit code 1.

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[Copilot]

Pull request overview

This PR fixes a build failure in the release workflow where raco exe -o randstr fails because randstr is an existing source directory, causing a "cannot delete file - Is a directory" error.

Key changes:

• Output executables to build/ subdirectory instead of repository root
• Split build step into platform-specific variants (Unix/Windows) with appropriate shell commands
• Update distribution commands to reference the new executable paths

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