Limit builds to 6 jobs (4 for arm)

The CI runners don't use fixed CPU cores anymore, so that they can
better allocate jobs across idle cores, but this means ninja's default
is running an insane number of parallel compilations that likely just
makes things slower.

Add 6-job limiting to match the core limits that used to be in place.

.drone.jsonnet

@@ -21,6 +21,7 @@ local debian_pipeline(name, image,
         werror=true,
         cmake_extra='',
         extra_cmds=[],
+        jobs=6,
         loki_repo=false,
         allow_fail=false) = {
     kind: 'pipeline',
@@ -55,7 +56,7 @@ local debian_pipeline(name, image,
                     (if werror then '-DWARNINGS_AS_ERRORS=ON ' else '') +
                     '-DWITH_LTO=' + (if lto then 'ON ' else 'OFF ') +
                 cmake_extra,
-                'ninja -v',
+                'ninja -j' + jobs + ' -v',
                 '../contrib/ci/drone-gdb.sh ./test/testAll --use-colour yes',
             ] + extra_cmds,
         }
@@ -93,6 +94,7 @@ local windows_cross_pipeline(name, image,
         cmake_extra='',
         toolchain='32',
         extra_cmds=[],
+        jobs=6,
         allow_fail=false) = {
     kind: 'pipeline',
     type: 'docker',
@@ -121,7 +123,7 @@ local windows_cross_pipeline(name, image,
                     (if lto then '' else '-DWITH_LTO=OFF ') +
                     "-DBUILD_STATIC_DEPS=ON -DDOWNLOAD_SODIUM=ON -DBUILD_PACKAGE=ON -DBUILD_SHARED_LIBS=OFF -DBUILD_TESTING=OFF -DNATIVE_BUILD=OFF -DSTATIC_LINK=ON" +
                 cmake_extra,
-                'ninja -v package',
+                'ninja -j' + jobs + ' -v package',
             ] + extra_cmds,
         }
     ],
@@ -178,7 +180,13 @@ local deb_builder(image, distro, distro_branch, arch='amd64', loki_repo=true) =
 
 
 // Macos build
-local mac_builder(name, build_type='Release', werror=true, cmake_extra='', extra_cmds=[], allow_fail=false) = {
+local mac_builder(name,
+        build_type='Release',
+        werror=true,
+        cmake_extra='',
+        extra_cmds=[],
+        jobs=6,
+        allow_fail=false) = {
     kind: 'pipeline',
     type: 'exec',
     name: name,
@@ -198,7 +206,7 @@ local mac_builder(name, build_type='Release', werror=true, cmake_extra='', extra
                 'cd build',
                 'cmake .. -G Ninja -DCMAKE_CXX_FLAGS=-fcolor-diagnostics -DCMAKE_BUILD_TYPE='+build_type+' ' +
                     (if werror then '-DWARNINGS_AS_ERRORS=ON ' else '') + cmake_extra,
-                'ninja -v',
+                'ninja -j' + jobs + ' -v',
                 './test/testAll --use-colour yes',
             ] + extra_cmds,
         }
@@ -233,8 +241,8 @@ local mac_builder(name, build_type='Release', werror=true, cmake_extra='', extra
                     cmake_extra='-DCMAKE_C_COMPILER=gcc-8 -DCMAKE_CXX_COMPILER=g++-8', loki_repo=true),
 
     // ARM builds (ARM64 and armhf)
-    debian_pipeline("Debian sid (ARM64)", "debian:sid", arch="arm64"),
-    debian_pipeline("Debian buster (armhf)", "arm32v7/debian:buster", arch="arm64", cmake_extra='-DDOWNLOAD_SODIUM=ON'),
+    debian_pipeline("Debian sid (ARM64)", "debian:sid", arch="arm64", jobs=4),
+    debian_pipeline("Debian buster (armhf)", "arm32v7/debian:buster", arch="arm64", cmake_extra='-DDOWNLOAD_SODIUM=ON', jobs=4),
     // Static armhf build (gets uploaded)
     debian_pipeline("Static (buster armhf)", "arm32v7/debian:buster", arch="arm64", deps='g++ python3-dev automake libtool',
                     cmake_extra='-DBUILD_STATIC_DEPS=ON -DBUILD_SHARED_LIBS=OFF -DSTATIC_LINK=ON ' +
@@ -243,7 +251,8 @@ local mac_builder(name, build_type='Release', werror=true, cmake_extra='', extra
                     extra_cmds=[
                         '../contrib/ci/drone-check-static-libs.sh',
                         'UPLOAD_OS=linux-armhf ../contrib/ci/drone-static-upload.sh'
-                    ]),
+                    ],
+                    jobs=4),
     // android apk builder
     apk_builder("android apk", "registry.oxen.rocks/lokinet-ci-android", extra_cmds=['UPLOAD_OS=anrdoid ../contrib/ci/drone-static-upload.sh']),