fix(chapel-ci): set CHPL_LLVM=none to skip LLVM backend build

Second cold-build attempt failed at:

  Error: Please set the environment variable CHPL_LLVM to a supported value.
    1) 'none' to build with minimal LLVM support
    2) 'bundled' ...
    3) 'system' ...

Chapel's compiler-builds tries to verify LLVM headers via
clang/Basic/Version.h before building; on Ubuntu 22.04 we don't have
LLVM dev headers installed, and we don't need them — CHPL_TARGET_COMPILER=gnu
already targets the C backend.

Setting CHPL_LLVM=none disables the LLVM backend entirely. Multilocale
GASNet+smp comms don't depend on LLVM, only on the runtime layer.

Author: hyperpolymath

.github/workflows/chapel-ci.yml

@@ -308,6 +308,11 @@ jobs:
           export CHPL_COMM_SUBSTRATE=smp
           export CHPL_LAUNCHER=smp
           export CHPL_TARGET_COMPILER=gnu
+          # CHPL_LLVM=none disables the LLVM backend (we only need the gnu C
+          # backend for the multilocale smoke test). Without this, the build
+          # tries to verify LLVM headers via clang/Basic/Version.h and aborts
+          # with a CHPL_LLVM "unset" make target.
+          export CHPL_LLVM=none
           # setchplenv.bash references ${MANPATH} unconditionally; GH runners
           # don't export MANPATH by default, so seed it before sourcing.
           export MANPATH="${MANPATH:-}"
@@ -322,6 +327,7 @@ jobs:
         run: |
           set -euo pipefail
           export CHPL_HOME="${{ env.CHAPEL_MULTILOCALE_HOME }}"
+          export CHPL_LLVM=none
           export MANPATH="${MANPATH:-}"
           source "$CHPL_HOME/util/setchplenv.bash"
           # Persist env to subsequent steps via GITHUB_ENV