feat: eic_ci_clang container

[wdconinc]

Briefly, what does this PR introduce?

Needs:

• Remove '%clang' requirement from several ACTS users #44
• chore: update (gcc,clang) to (14,20); add flang #88

Tests in GitHub workflows of epic and EICrecon indicate faster and smaller code when compiled with clang instead of gcc. This PR prepares a stack to run some tests with.

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue: eic_ci_clang)
• Documentation update
• Other: __

[Copilot]

Pull Request Overview

This PR introduces a new ci_clang container environment to test compilation with the Clang compiler, which has shown performance benefits in GitHub workflow testing. The changes configure Spack to build a comprehensive stack using Clang instead of GCC.

Key Changes:

• Added new ci_clang spack environment configured to build with Clang compiler and enable inter-procedural optimization (IPO)
• Configured MPI to use mpich without Fortran support to better accommodate Clang compilation
• Enabled mixed-toolchain compiler discovery in Docker builds to support both Clang and GCC compilers

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
spack-environment/packages.yaml	Adds MPI configuration requiring mpich 4.2.1 without Fortran support to accommodate Clang builds
spack-environment/ci_clang/spack.yaml	New spack environment configuration that requires Clang compiler, enables IPO where available, and includes comprehensive package list for CI testing
containers/debian/Dockerfile	Adds --mixed-toolchain flag to compiler discovery to support mixed Clang/GCC toolchain usage
.gitlab-ci.yml	Adds ci_clang to the nightly build matrix for the eic environment

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[Copilot]

The py-scipy package is missing from this environment but is present in the comparable ci and ci_without_acts environments (both include py-scipy at similar positions in their specs lists). Consider adding - py-scipy after py-rucio-clients for consistency with other CI environments.

Suggested change

	- py-rucio-clients
	- py-rucio-clients
	- py-scipy