build: revert back to go1.12 for the v1.49.x builds

The go1.13 build has had various problems reported so revert back to
go1.12 for the stable branch.

See: https://forum.rclone.org/t/1-49-4-plex-internal-errors-on-google-drive/12108
Fixes #3578

.github/workflows/build.yml

@@ -24,7 +24,7 @@ jobs:
         include:
           - job_name: linux
             os: ubuntu-latest
-            go: '1.13.x'
+            go: '1.12.x'
             modules: 'off'
             gotags: cmount
             build_flags: '-include "^linux/"'
@@ -34,7 +34,7 @@ jobs:
 
           - job_name: mac
             os: macOS-latest
-            go: '1.13.x'
+            go: '1.12.x'
             modules: 'off'
             gotags: ''  # cmount doesn't work on osx travis for some reason
             build_flags: '-include "^darwin/amd64" -cgo'
@@ -44,7 +44,7 @@ jobs:
 
           - job_name: windows_amd64
             os: windows-latest
-            go: '1.13.x'
+            go: '1.12.x'
             modules: 'off'
             gotags: cmount
             build_flags: '-include "^windows/amd64" -cgo'
@@ -54,7 +54,7 @@ jobs:
 
           - job_name: windows_386
             os: windows-latest
-            go: '1.13.x'
+            go: '1.12.x'
             modules: 'off'
             gotags: cmount
             goarch: '386'
@@ -65,15 +65,15 @@ jobs:
 
           - job_name: other_os
             os: ubuntu-latest
-            go: '1.13.x'
+            go: '1.12.x'
             modules: 'off'
             build_flags: '-exclude "^(windows/|darwin/amd64|linux/)"'
             compile_all: true
             deploy: true
 
           - job_name: modules_race
             os: ubuntu-latest
-            go: '1.13.x'
+            go: '1.12.x'
             modules: 'on'
             quicktest: true
             racequicktest: true
@@ -90,12 +90,6 @@ jobs:
             modules: 'off'
             quicktest: true
 
-          - job_name: go1.12
-            os: ubuntu-latest
-            go: '1.12.x'
-            modules: 'off'
-            quicktest: true
-
     name: ${{ matrix.job_name }}
 
     runs-on: ${{ matrix.os }}