firejail: disable parallel building

firejail was frequently failing to build on my Hydra machine at -j16, and
the error looked like a typical parallel build problem:

<3>make[1]: Entering directory '/build/firejail-0.9.56/src/fcopy'
<3>gcc -ggdb  -O2 -DVERSION='"0.9.56"'   -DPREFIX='"/nix/store/0dm1agiwiggn8pmnqkknil7mkh25il0k-firejail-0.9.56"'  -DSYSCONFDIR='"/nix/store/0dm1agiwiggn8pmnqkknil7mkh25il0k-firejail-0.9.56/etc/firejail"' -DLIBDIR='"/nix/store/0dm1agiwiggn8pmnqkknil7mkh25il0k-firejail-0.9.56/lib"' -DHAVE_X11 -DHAVE_PRIVATE_HOME  -DHAVE_OVERLAYFS -DHAVE_SECCOMP -DHAVE_GLOBALCFG -DHAVE_SECCOMP_H -DHAVE_CHROOT -DHAVE_NETWORK -DHAVE_USERNS -DHAVE_FILE_TRANSFER -DHAVE_WHITELIST -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIE -pie -Wformat -Wformat-security -mindirect-branch=thunk   -c main.c -o main.o
<3>gcc  -pie -Wl,-z,relro -Wl,-z,now -lpthread -o fcopy main.o
<3>make[1]: Leaving directory '/build/firejail-0.9.56/src/fcopy'
<3>make -C src/fldd
<3>make[1]: Entering directory '/build/firejail-0.9.56/src/fldd'
<3>gcc -ggdb  -O2 -DVERSION='"0.9.56"'   -DPREFIX='"/nix/store/0dm1agiwiggn8pmnqkknil7mkh25il0k-firejail-0.9.56"'  -DSYSCONFDIR='"/nix/store/0dm1agiwiggn8pmnqkknil7mkh25il0k-firejail-0.9.56/etc/firejail"' -DLIBDIR='"/nix/store/0dm1agiwiggn8pmnqkknil7mkh25il0k-firejail-0.9.56/lib"' -DHAVE_X11 -DHAVE_PRIVATE_HOME  -DHAVE_OVERLAYFS -DHAVE_SECCOMP -DHAVE_GLOBALCFG -DHAVE_SECCOMP_H -DHAVE_CHROOT -DHAVE_NETWORK -DHAVE_USERNS -DHAVE_FILE_TRANSFER -DHAVE_WHITELIST -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIE -pie -Wformat -Wformat-security -mindirect-branch=thunk   -c main.c -o main.o
<3>gcc  -pie -Wl,-z,relro -Wl,-z,now -lpthread -o fldd main.o ../lib/ldd_utils.o
<3>make[1]: Leaving directory '/build/firejail-0.9.56/src/fldd'
<3>make -C src/libpostexecseccomp
<3>make[1]: Entering directory '/build/firejail-0.9.56/src/libpostexecseccomp'
<3>gcc -ggdb  -O2 -DVERSION='"0.9.56"' -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIC -Wformat -Wformat-security  -c libpostexecseccomp.c -o libpostexecseccomp.o
<3>gcc -pie -Wl,-z,relro -Wl,-z,now -shared -fPIC -z relro -o libpostexecseccomp.so libpostexecseccomp.o -ldl
<3>make[1]: Leaving directory '/build/firejail-0.9.56/src/libpostexecseccomp'
<3>src/fseccomp/fseccomp default seccomp
<3>src/fsec-optimize/fsec-optimize seccomp
<3>/nix/store/6abyjgibafsbhlc7v7lab50mb3dj81jg-bash-4.4-p23/bin/bash: src/fsec-optimize/fsec-optimize: No such file or directory
<3>make: *** [Makefile:43: filters] Error 127
<3>builder for '/nix/store/30srqmpqrjyr11nhx4jbpr84m9pnmyv5-firejail-0.9.56.drv' failed with exit code 2