Merge bitcoin#28504: ci: Use nproc over MAKEJOBS in 01_base_install

fa3b5e5 ci: Use nproc over MAKEJOBS in 01_base_install (MarcoFalke)

Pull request description:

  Currently `$MAKEJOBS` is the default value in `01_base_install.sh` when building the container image.

  This problem can't be fixed (see below), so just use `nproc` for now.

  Other solutions would be bad:

  * Passing in the `MAKEJOBS` as a dockerfile env would create a new image if the number of tasks are changed, seems verbose and confusing.
  * Leaving `master` as-is would leave CPUs unused if there are more than `4`.

ACKs for top commit:
  fanquake:
    ACK - I think this is fine as-is, it's an improvement over the current state fa3b5e5

Tree-SHA512: 15014eb7b548945737b0fed5cd46f0cd4b72b1d54d044f60fce628f914053a2de6518878428a54822d236d08d6f257d8c464d3fb589400f4be56022d2ec8676d

ci/test/01_base_install.sh

@@ -51,7 +51,7 @@ if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then
     -DLLVM_ENABLE_RUNTIMES="compiler-rt;libcxx;libcxxabi;libunwind" \
     -S /msan/llvm-project/llvm
 
-  ninja -C /msan/clang_build/ "$MAKEJOBS"
+  ninja -C /msan/clang_build/ "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
   ninja -C /msan/clang_build/ install-runtimes
 
   update-alternatives --install /usr/bin/clang++ clang++ /msan/clang_build/bin/clang++ 100
@@ -69,13 +69,13 @@ if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then
     -DLIBCXX_HARDENING_MODE=debug \
     -S /msan/llvm-project/runtimes
 
-  ninja -C /msan/cxx_build/ "$MAKEJOBS"
+  ninja -C /msan/cxx_build/ "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
 fi
 
 if [[ "${RUN_TIDY}" == "true" ]]; then
   git clone --depth=1 https://github.com/include-what-you-use/include-what-you-use -b clang_16 /include-what-you-use
   cmake -B /iwyu-build/ -G 'Unix Makefiles' -DCMAKE_PREFIX_PATH=/usr/lib/llvm-16 -S /include-what-you-use
-  make -C /iwyu-build/ install "$MAKEJOBS"
+  make -C /iwyu-build/ install "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
 fi
 
 mkdir -p "${DEPENDS_DIR}/SDKs" "${DEPENDS_DIR}/sdk-sources"