perf(gopls): lazy cache initialization

[cljoly]

On my machine, prior to this patch, gopls requires more than 38ms to
load (even after multiple starts of neovim):

077.729  038.355  038.355: require('lspconfig.server_configurations.gopls')

while the other servers take tens of millisecond.

After implementing this change, gopls takes about the same time as the
others:

033.563  000.163  000.163: require('lspconfig.server_configurations.gopls')

I have tried to replace go env GOMODCACHE with
echo /home/cjoly/go/pkg/mod but it’s just as slow. I suspect that the
call to system() is so slow because I use the fish shell. If I change
the 'shell' setting to 'sh', the issue disappears. Nonetheless, it’s
probably worth fixing, since this change is not very complex IMO, even
if my configuration is somewhat exotic.

I’ve checked that this change does not make using the 'sh' shell slower
and that the LSP still initializes properly.

[glepnir]

if this command have performance issue can make it async in root_dir .

[glepnir]

I have make it async in #2673 could you test it

[cljoly]

Just seen your second comment, thank you!

Yes, your solution in #2673 does fix the issue as well.