Build failed with "database is locked"

[sogaiu]

I had 3 simultaneous builds going (2 of them saying "waiting for more work") -- the one that was proceeding ended like this:

'/hpkg/bf64527dd013370adf1245ca6ba431f817588a50-mcm-gcc/x86_64-linux-musl' -> '/hpkg/e81eb6be19054d22b46ff705aa1d99e786ec9cf5-gcc/x86_64-linux-musl'
error: database is locked
  in sqlite3/eval
  in run-builder [src/pkgstore.janet] on line 624, column 9
  in <anonymous> [src/pkgstore.janet] on line 458, column 21
  in build-pkg [src/pkgstore.janet] on line 449, column 17
  in build-pkg [src/pkgstore.janet] on line 445, column 36
  in build-pkg [src/pkgstore.janet] on line 445, column 36
  in <anonymous> [src/pkgstore.janet] on line 629, column 13
  in <anonymous> [src/pkgstore.janet] on line 433, column 3
  in <anonymous> [src/pkgstore.janet] on line 432, column 3
  in build [src/pkgstore.janet] on line 431, column 3
  in build [src/hermes-pkgstore-main.janet] on line 171, column 3
  in <anonymous> [src/hermes-pkgstore-main.janet] on line 288, column 19
  in main [src/hermes-pkgstore-main.janet] on line 285, column 3
Command exited with non-zero status 1