siso: fix data race in abfs_test

--- FAIL: TestBuild_ABFS (0.25s)
    abfs_test.go:40:
        2026/08/13 11:18:20 INFO validated blobs in CAS count=1 size="0 MiB" duration=604.1µs hash_speed="0.00 MiB/s"
        2026/08/13 11:18:20 INFO validated results in action cache count=0 referenced_blobs=0 duration=94.94µs
        schedule pending:1+ready:0 (node:4 edge:1)
        build start: Ready 1 Pending 0
        [0/1] 0.00s S python3 ../../tools/cp.py ../../foo.in gen/foo.out
        fatal error: concurrent map writes

        goroutine 513 [running]:
        internal/runtime/maps.fatal({0x15179c2?, 0x12fc1c0?})
        	/usr/lib/go-1.26/src/runtime/panic.go:1181 +0x18
        go.chromium.org/build/siso/subcmd/ninja/e2etests.TestBuild_ABFS.func3({0x15c4950, 0x300237974ff0}, 0x300237d1bb30?)
        	/usr/local/google/home/ukai/src/crbuild/build/siso/subcmd/ninja/e2etests/abfs_test.go:106 +0xfd
        net/http.HandlerFunc.ServeHTTP(0x300237b14000?, {0x15c4950?, 0x300237974ff0?}, 0x7c59b6?)
        	/usr/lib/go-1.26/src/net/http/server.go:2286 +0x29
        net/http.(*ServeMux).ServeHTTP(0x48aab9?, {0x15c4950, 0x300237974ff0}, 0x300237a077c0)
        	/usr/lib/go-1.26/src/net/http/server.go:2828 +0x1c7
        net/http.serverHandler.ServeHTTP({0x300237a05740?}, {0x15c4950?, 0x300237974ff0?}, 0x1?)
        	/usr/lib/go-1.26/src/net/http/server.go:3311 +0x8e
        net/http.(*conn).serve(0x300237c90090, {0x15c8388, 0x3002376e8240})
        	/usr/lib/go-1.26/src/net/http/server.go:2073 +0x650
        created by net/http.(*Server).Serve in goroutine 193
        	/usr/lib/go-1.26/src/net/http/server.go:3464 +0x485
    abfs_test.go:40: subprocess test failed: exit status 2
FAIL

Change-Id: I9bc1cff5f61f7381e06a37ef6cf4d0e9132ee2ce
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8256691
Reviewed-by: Yuke Liao <liaoyuke@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>