lib.meta: Avoid attrset allocation in platformMatch
Benchmarks (`nix-instantiate ./. -A python3`)
- Before
``` json
{
"cpuTime": 0.30625399947166443,
"envs": {
"bytes": 4484216,
"elements": 221443,
"number": 169542
},
"gc": {
"heapSize": 402915328,
"totalBytes": 53091024
},
"list": {
"bytes": 749424,
"concats": 4242,
"elements": 93678
},
"nrAvoided": 253991,
"nrFunctionCalls": 149848,
"nrLookups": 49614,
"nrOpUpdateValuesCopied": 1588326,
"nrOpUpdates": 10106,
"nrPrimOpCalls": 130356,
"nrThunks": 359013,
"sets": {
"bytes": 30432320,
"elements": 1860540,
"number": 41480
},
"sizes": {
"Attr": 16,
"Bindings": 16,
"Env": 16,
"Value": 24
},
"symbols": {
"bytes": 236218,
"number": 24459
},
"values": {
"bytes": 10504632,
"number": 437693
}
}
```
- After
```
{
"cpuTime": 0.29695799946784973,
"envs": {
"bytes": 3296712,
"elements": 169055,
"number": 121517
},
"gc": {
"heapSize": 402915328,
"totalBytes": 49044992
},
"list": {
"bytes": 504928,
"concats": 4242,
"elements": 63116
},
"nrAvoided": 175403,
"nrFunctionCalls": 110554,
"nrLookups": 44907,
"nrOpUpdateValuesCopied": 1588326,
"nrOpUpdates": 10106,
"nrPrimOpCalls": 82330,
"nrThunks": 306625,
"sets": {
"bytes": 29943328,
"elements": 1843076,
"number": 28382
},
"sizes": {
"Attr": 16,
"Bindings": 16,
"Env": 16,
"Value": 24
},
"symbols": {
"bytes": 236218,
"number": 24459
},
"values": {
"bytes": 9037752,
"number": 376573
}
}
```
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