guix: Use --cores instead of --max-jobs

In Guix, there are two flags for controlling parallelism:

Note: When I say "derivation," think "package"

--cores=n
  - controls the number of CPU cores to build each derivation. This is
    the value passed to `make`'s `--jobs=` flag.
  - defaults to 0: as many cores as is available

--max-jobs=n
  - controls how many derivations can be built in parallel
  - defaults to 1

Therefore, if set --max-jobs=$MAX_JOBS and don't set --cores, Guix could
theoretically spin up $MAX_JOBS * $(nproc) number of threads, and that's
no good.

So we could either default to --cores=1, --max-jobs=$MAX_JOBS

  - Pro: --cores=1 means that `make` will be invoked with `-j1`,
         avoiding problems with package whose build systems and test
         suites break when running multi-threaded.

  - Con: There will be times when only 1 or 2 derivations can be built
         at a time, because the rest of the dependency graph all depend
         on those 1 or 2 derivations. During these times, the machine
         will be severely under-utilized.

or --cores=$MAX_JOBS, --max-jobs=1

  - Pro: We don't encounter prolonged periods of
         severe under-utilization mentioned above.

  - Con: Many packages' build systems and test suites break when running
         multi-threaded.

or --cores=1, --max-jobs=1 and let the user override with
$ADDITIONAL_GUIX_COMMON_FLAGS

contrib/guix/libexec/build.sh

@@ -26,7 +26,7 @@ cat << EOF
 Required environment variables as seen inside the container:
     HOST: ${HOST:?not set}
     SOURCE_DATE_EPOCH: ${SOURCE_DATE_EPOCH:?not set}
-    MAX_JOBS: ${MAX_JOBS:?not set}
+    JOBS: ${JOBS:?not set}
     DISTSRC: ${DISTSRC:?not set}
     OUTDIR: ${OUTDIR:?not set}
 EOF
@@ -173,7 +173,7 @@ esac
 ####################
 
 # Build the depends tree, overriding variables that assume multilib gcc
-make -C depends --jobs="$MAX_JOBS" HOST="$HOST" \
+make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \
                                    ${SOURCES_PATH+SOURCES_PATH="$SOURCES_PATH"} \
                                    ${BASE_CACHE+BASE_CACHE="$BASE_CACHE"} \
@@ -267,7 +267,7 @@ mkdir -p "$DISTSRC"
     sed -i.old 's/-lstdc++ //g' config.status libtool src/univalue/config.status src/univalue/libtool
 
     # Build Bitcoin Core
-    make --jobs="$MAX_JOBS" ${V:+V=1}
+    make --jobs="$JOBS" ${V:+V=1}
 
     # Perform basic ELF security checks on a series of executables.
     make -C src --jobs=1 check-security ${V:+V=1}
@@ -344,7 +344,7 @@ mkdir -p "$DISTSRC"
                 {
                     find "${DISTNAME}/bin" -type f -executable -print0
                     find "${DISTNAME}/lib" -type f -print0
-                } | xargs -0 -n1 -P"$MAX_JOBS" -I{} "${DISTSRC}/contrib/devtools/split-debug.sh" {} {} {}.dbg
+                } | xargs -0 -n1 -P"$JOBS" -I{} "${DISTSRC}/contrib/devtools/split-debug.sh" {} {} {}.dbg
                 ;;
         esac