Get rid of most .at calls (#6393)

Use one of `get` or `getOr` instead which will either return a null-pointer (with a nicer error message) or a default value when the key is missing.

src/libexpr/get-drvs.cc

@@ -34,7 +34,7 @@ DrvInfo::DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPat
 
     outputName =
         selectedOutputs.empty()
-        ? get(drv.env, "outputName").value_or("out")
+        ? getOr(drv.env, "outputName", "out")
         : *selectedOutputs.begin();
 
     auto i = drv.outputs.find(outputName);