syncthing: pin go@1.25

[stefanb]

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Ref:

• go 1.26.0, go@1.25 1.25.7 (new formula) #258912
• runtime go1.26rc1 not found in compat.yaml syncthing/syncthing#10518
• chore: build with Go 1.26; use Go 1.25 features syncthing/syncthing#10570
• https://github.com/syncthing/syncthing/releases/tag/v2.0.15-rc.1