I found these instructions:
https://github.com/0ad/0ad/blob/master/binaries/data/config/default.cfg
I created a file ~/.config/0ad/config/local.cfg with this content:
windowed = true
That was ingnored; the game was still in fullscreen. I then found, that in my environment there is no XDG_CONFIG_HOME variable, so I set it to ~/.config.
This is the console output, when I run the game now:
CreateDirectories: failed to mkdir ~/.config/0ad/logs (mode 448)
file_system.cpp(158): Function call failed: return value was -110300 (Insufficient access rights to open file)
debug.cpp(189): Unable to open crashlog.txt for writing (please ensure the log directory is writable)
Unable to open crashlog.txt for writing (please ensure the log directory is writable)
Location: debug.cpp:189 (debug_WriteCrashlog)
Call stack:
(0x9da59e) /snap/0ad/18/binaries/system/pyrogenesis() [0x9da59e]
(0x976fc9) /snap/0ad/18/binaries/system/pyrogenesis() [0x976fc9]
(0x977395) /snap/0ad/18/binaries/system/pyrogenesis() [0x977395]
(0x978a31) /snap/0ad/18/binaries/system/pyrogenesis() [0x978a31]
(0x9773a2) /snap/0ad/18/binaries/system/pyrogenesis() [0x9773a2]
(0x977718) /snap/0ad/18/binaries/system/pyrogenesis() [0x977718]
(0x97d70f) /snap/0ad/18/binaries/system/pyrogenesis() [0x97d70f]
(0x97dc40) /snap/0ad/18/binaries/system/pyrogenesis() [0x97dc40]
(0x68c858) /snap/0ad/18/binaries/system/pyrogenesis() [0x68c858]
(0x432677) /snap/0ad/18/binaries/system/pyrogenesis() [0x432677]
(0x422447) /snap/0ad/18/binaries/system/pyrogenesis() [0x422447]
(0x7f6e82d6c830) /snap/0ad/18/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f6e82d6c830]
(0x4303b9) /snap/0ad/18/binaries/system/pyrogenesis() [0x4303b9]
errno = 0 (?)
OS error = ?
(C)ontinue, (B)reak, Launch (D)ebugger, or (E)xit?