ci(rumdl): cap CARGO_BUILD_JOBS=2 to fix riscv64 SIGABRT

The 0.2.75 build leg died compiling rumdl_lib itself with a glibc
heap-corruption abort ("corrupted double-linked list") -> rustc
signal 6 (SIGABRT) -> "Cargo build finished with exit status: 101",
while the 0.2.76 leg succeeded compiling the identical
tikv-jemalloc-sys/tikv-jemallocator v0.6.1 pin at the same time on
the same 4-core riscv64 runner fleet. This matches gotcha 228: default
cargo parallelism on this fleet can SIGABRT rustc under memory
pressure when two heavy matrix legs compile concurrently, not a real
dependency incompatibility. Same fix already used for
build-deltalite.yml/build-connectorx.yml.

Author: luhenry

.github/workflows/build-rumdl.yml

@@ -30,6 +30,17 @@ concurrency:
 permissions:
   contents: read  # to fetch code (actions/checkout)
 
+env:
+  # Halves concurrent rustc processes on the 4-core riscv64 runner (gotcha
+  # 228). rumdl_lib's own final compile is the heaviest step (blake3, chrono,
+  # tokio, tower-lsp, rayon, notify, ... all enabled), and the 0.2.75 leg
+  # SIGABRT'd there with a glibc heap-corruption abort ("corrupted
+  # double-linked list") while the 0.2.76 leg was concurrently compiling the
+  # same crate on the same runner fleet -- both matrix legs' Cargo.lock pin
+  # the identical tikv-jemalloc-sys/tikv-jemallocator v0.6.1, so this is
+  # memory contention at default parallelism, not a dependency regression.
+  CARGO_BUILD_JOBS: 2
+
 jobs:
   setup:
     uses: $/.github/workflows/_setup.yml