feat: custom build folder

[perazz]

This PR adds support for specifying a custom build/ folder for build outputs in fpm. Users can now configure the build directory using either the --build-dir command-line option or the FPM_BUILD_DIR environment variable.

So, builds with different CLI/environment features (i.e. compilers, etc.) can have their own folder, avoiding overwrites.

Example:

fpm build --build-dir build_gcc
fpm install --build-dir build_gcc --prefix=/gcc_packages/

Closes #1137.

[Copilot]

Pull Request Overview

This PR implements support for custom build directories in the fpm (Fortran Package Manager) to allow users to specify different output locations for build artifacts. This enables parallel builds with different configurations without overwrites.

• Adds --build-dir command-line option and FPM_BUILD_DIR environment variable support
• Updates the fpm model to track and validate custom build directory paths
• Implements validation to prevent conflicts with reserved source directory names

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/fpm_model.f90	Adds build_dir field to model type with serialization support
src/fpm_command_line.f90	Implements CLI parsing and validation for build directory option
src/fpm_backend_output.f90	Updates build progress tracking to use custom build directory
src/fpm_backend.F90	Passes build directory to progress initialization
src/fpm/dependency.f90	Updates dependency tree to use custom build directory
src/fpm/cmd/update.f90	Modifies update command to respect custom build directory
src/fpm/cmd/export.f90	Updates export command for custom build directory support
src/fpm.f90	Integrates build directory into main build model logic
ci/test_custom_build_dir.sh	Comprehensive test suite for custom build directory functionality
ci/run_tests.sh	Integrates custom build directory tests into CI pipeline

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