Avoid an error with long-decimal token prices

When an Ethereum token price has more decimals than the fiatDecimals,
BigInt would receive a floating-point number and error out; this commit
adds a call to Math.trunc to truncate further precision (not round!).

background/lib/prices.ts

@@ -139,7 +139,7 @@ export async function getEthereumTokenPrices(
           symbol: currencySymbol.toUpperCase(),
           decimals: fiatDecimals,
         },
-        amount: BigInt(price * 10 ** fiatDecimals),
+        amount: BigInt(Math.trunc(price * 10 ** fiatDecimals)),
       },
       time: priceDetails.last_updated_at,
     }