I think that was an intention.
I will repeat citation from here, sorry:
Shells for instance, map environment variables to shell variables and are likely not to use putenv/setenv libc functions
I came across this interesting recent post about thread safety of setenv, which might be worth considering: Setenv is not Thread Safe and C Doesn't Want to Fix It (evanjones.ca)
FWIW I’m +1 on having a setenv function available - although env vars can cause gotchas and hidden behaviour changes, in some cases it’s a sensible/practical tool.
Zig does map the environ variable to std.c.environ, so you could write your own function to modify it. Or you can link libc and add your own extern to use it:
extern fn putenv(string: [*:0]u8) c_int;
If you want it to be included in the standard library, then perhaps an issue should be made on the Ziglang git repo?