Fix Windows path conversion in .envrc creation

Convert Windows-style paths from setup-java outputs to Unix paths
using cygpath. The setup-java action returns Windows paths like
'C:\hostedtoolcache\...' on Windows, which need to be converted to
Unix-style paths '/c/hostedtoolcache/...' for bash scripts.

Changes:
- Add conditional cygpath conversion for Windows runner
- Convert JAVA8_PATH, JAVA11_PATH, JAVA17_PATH on Windows only
- Leave paths unchanged on Linux and macOS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Introduced in the course of support-and-care/maven-support-and-care#137

Author: ascheman

.github/workflows/build.yml

@@ -79,6 +79,13 @@ jobs:
         JAVA11_PATH: ${{ steps.setup-jdk11.outputs.path }}
         JAVA17_PATH: ${{ steps.setup-jdk17.outputs.path }}
       run: |
+        # Convert Windows paths to Unix paths if on Windows
+        if [[ "$RUNNER_OS" == "Windows" ]]; then
+          JAVA8_PATH="$(cygpath -u "$JAVA8_PATH")"
+          JAVA11_PATH="$(cygpath -u "$JAVA11_PATH")"
+          JAVA17_PATH="$(cygpath -u "$JAVA17_PATH")"
+        fi
+
         cat > .envrc << EOF
         # Environment configuration for Java 9 Jigsaw Examples
         # Auto-generated by GitHub Actions