travis+lint: fix memory usage

There is a setting to control how often the garbage collector is run.
Apparently this is a tradeoff between CPU and memory usage. If we can
limit the memory being used in that way, this allows us to use multiple
worker again, so overall this shouldn't be much slower than before.

.golangci.yml

@@ -1,6 +1,6 @@
 run:
   # timeout for analysis
-  deadline: 4m
+  deadline: 10m
 
   # Skip autogenerated files for mobile.
   skip-files:

.travis.yml

@@ -44,8 +44,8 @@ jobs:
         # in a GitHub Workflow.
         - make unit pkg=... case=_NONE_
 
-        # Step 3: Lint go code. Limit to 1 worker to reduce memory usage.
-        - make lint workers=1
+        # Step 3: Lint go code. Invoke GC more often to reduce memory usage.
+        - GOGC=30 make lint
 
     - stage: Integration Test
       name: Btcd Integration