main.star: set GOEXPERIMENT=synctest for x/net builders testing >=go1.24

Also, treat GOEXPERIMENT in the run mod definition as additive.
Otherwise this new setting will be overridden by any builder that sets a
GOEXPERIMENT via a run mod.

This is a re-land of CL 642422 which was reverted because it broke
builders running old toolchains, even though the build targeted a new
release. (These builders should probably explicitly unset GOEXPERIMENT
when running these toolchains, but that's a longer-term fix. x/tools has
already been fixed in this regard.)

This change shouldn't break anything because it only applies the changes
in CL 642422 to the x/net builders, and those builders were fine.

For golang/go#67434.

Change-Id: Ifa8ec159157721afd694f8afdcb0b58795cf6787
Reviewed-on: https://go-review.googlesource.com/c/build/+/642675
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>