Fix UNIX socket file too long

Why do we need this MR

My system version is macOS 11.4 .

I tried to execute a go test, got FAIL like this:

test command and log:

GOROOT=/Users/cbd/.asdf/installs/golang/1.16.4/go #gosetup
GOPATH=/Users/cbd/go #gosetup
/Users/cbd/.asdf/installs/golang/1.16.4/go/bin/go test -c -o /private/var/folders/yn/9bkfy_zs6nx881snv4_t_44c0000gn/T/___TestUserCommitFiles_in_gitlab_com_gitlab_org_gitaly_v14_internal_gitaly_service_operations.test gitlab.com/gitlab-org/gitaly/v14/internal/gitaly/service/operations #gosetup
/Users/cbd/.asdf/installs/golang/1.16.4/go/bin/go tool test2json -t /private/var/folders/yn/9bkfy_zs6nx881snv4_t_44c0000gn/T/___TestUserCommitFiles_in_gitlab_com_gitlab_org_gitaly_v14_internal_gitaly_service_operations.test -test.v -test.paniconexit0 -test.run ^\QTestUserCommitFiles\E$
=== RUN   TestUserCommitFiles
    gitaly_builder.go:151: 
        	Error Trace:	gitaly_builder.go:125
        	            				gitaly_builder.go:151
        	            				testhelper_test.go:73
        	            				commit_files_test.go:36
        	Error:      	Received unexpected error:
        	            	socket could not be created in /var/folders/yn/9bkfy_zs6nx881snv4_t_44c0000gn/T/gitaly-801540891/519633854/internal_socks.d: listen unix /var/folders/yn/9bkfy_zs6nx881snv4_t_44c0000gn/T/gitaly-801540891/519633854/internal_socks.d/test-fd848dac.sock: bind: invalid argument

Although the message here is bind: invalid argument, the real reason is that the unix socket file path is too long.

Here, ioutil.TempDir returned to a macOS-style temporary directory: /var/folders/yn/9bkfy_zs6nx881snv4_t_44c0000gn/T/. It's too long.

By the way, we can find the prompt from the man unix:

UNIX-domain addresses are variable-length filesystem pathnames of at most 104 characters.

Why did I do this

After modification, we will get such a temporary directory: /tmp/gitaly-238865111 .

It's short and has good compatibility.