Fix go tip unit test runs (jaegertracing#5044)

## Which problem is this PR solving?
- Resolves jaegertracing#5043

## Description of the changes
- `go-junit-report` was getting installed successfully into the default
`$GOPATH` (`$HOME/go/bin`) but the GOPATH wasn't in the `$PATH`
environment variable, which is what caused the `go-junit-report: command
not found` error.
- It worked for the normal (non-go-tip) unit tests because the `$GOPATH`
was already set and inserted into `$PATH`.
- This explicitly sets the `$GOPATH` and inserts it into `$PATH` prior
to installing test tools.

## How was this change tested?
- Tested in [my own
fork](https://github.com/albertteoh/jaeger/actions/runs/7332463204/job/19966641893)
with some debug logging in the Makefile.

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits

---------

Signed-off-by: Albert Teoh <albert@packsmith.io>
Co-authored-by: Albert Teoh <albert@packsmith.io>

.github/actions/setup-go-tip/action.yml

@@ -30,10 +30,12 @@ runs:
         echo "Downloaded bundle:"
         ls -lah gotip.tar.gz
         export GOROOT="$HOME/sdk/gotip"
+        export GOPATH="$HOME/go"
         mkdir -p $GOROOT
         tar -C $GOROOT -xzf gotip.tar.gz
-        export PATH="$GOROOT/bin/:$PATH"
+        export PATH="$GOROOT/bin/:$GOPATH/bin:$PATH"
         echo "GOROOT=$GOROOT" >> $GITHUB_ENV
+        echo "GOPATH=$GOPATH" >> $GITHUB_ENV
         echo "PATH=$PATH" >> $GITHUB_ENV
         echo "Active Go version:"
-        go version
+        go version