builder: fix json-errors windows path separator

[kbkpbot]

This PR fix json-errors message which will pass to vls.

This bug will cause json decode fail when the path is a windows path separator.

js=[
{
"path":"C:\Users\DDT\AppData\Local\Temp\atof.v",
"message":"unknown field `no_extra_char` in struct literal of type `strconv.AtoF64Param`.\nDid you mean `allow_extra_chars`?",
"line_nr":11,
"col":27,
"len":0
}
]

failed to parse json failed to decode JSON string: {
"path":"C:\Users\DDT\AppDat
run_v_check errors:[]

[kbkpbot]

with vlang/vls#472, it make zed or vscode work under windows and linux.

[spytheman]

Thanks @kbkpbot .