fix: point Bitcoin icon URLs at pending/post-fix-release, not master

Master never got the Bitcoin icon files (they landed only on
pending/post-fix-release, same branch CHAINS_URL is currently testing
against) - Tron's icon happened to already exist on master from an
earlier, unrelated commit, so it worked by coincidence while Bitcoin's
404'd. Confirmed live: master/icons/tokens/colored/BTC.svg -> 404,
pending/post-fix-release/... -> 200.

Same caveat as the CHAINS_URL override itself: revert to master once
this lands there in the coordinated release.

chains/v21/chains.json

@@ -297,7 +297,7 @@
   {
     "chainId": "bip122:000000000019d6689c085ae165831e93",
     "name": "Bitcoin",
-    "icon": "https://raw.githubusercontent.com/pezkuwichain/pezkuwi-wallet-utils/master/icons/chains/gradient/Bitcoin.svg",
+    "icon": "https://raw.githubusercontent.com/pezkuwichain/pezkuwi-wallet-utils/pending/post-fix-release/icons/chains/gradient/Bitcoin.svg",
     "addressPrefix": 0,
     "options": [
       "bitcoinBased",
@@ -327,7 +327,7 @@
         "priceId": "bitcoin",
         "staking": null,
         "type": "bitcoinNative",
-        "icon": "https://raw.githubusercontent.com/pezkuwichain/pezkuwi-wallet-utils/master/icons/tokens/colored/BTC.svg",
+        "icon": "https://raw.githubusercontent.com/pezkuwichain/pezkuwi-wallet-utils/pending/post-fix-release/icons/tokens/colored/BTC.svg",
         "buyProviders": {},
         "sellProviders": {},
         "typeExtras": null