Use goproxy to make go mode download work on centos7 (old git version)

Alternative solution was to update git on centos7 in /rpm/SPEC/docker-ce-cli.spec: 
```
if [ "$SUITE" == "7" ]; then
    yum install -y http://opensource.wandisco.com/centos/7/git/x86_64/wandisco-git-release-7-2.noarch.rpm
    yum install -y git
fi
```

But rather not changing what is installed on the CEnTOS bistro for the entire CI pipeline

Signed-off-by: Guillaume Tardif <[email protected]>

Author: gtardif

plugins/scan.installer

@@ -17,7 +17,11 @@ build() {
         cd "${DEST}"
         git fetch --all
         git checkout -q "${COMMIT}"
-        PLATFORM_BINARY=docker-scan make native-build
+        # TODO remove GOPROXY override once we updated to Go 1.14+
+        # Using goproxy instead of "direct" to work around an issue in go mod
+        # on Go 1.13 not working with older git versions (default version on
+        # CentOS 7 is git 1.8), see https://github.com/golang/go/issues/38373
+        GOPROXY="https://proxy.golang.org" PLATFORM_BINARY=docker-scan make native-build
     )
 }