cache: return error if cacheservice scheme is not supported

[marcosnils]

otherwise we get a nasty nil pointer exception:

time="2023-05-26T17:05:16Z" level=debug msg="using cache service at localhost:8020/magicache"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x6e0fd4]

goroutine 1 [running]:
net/http.(*Client).deadline(0xc000665520?)
        /usr/local/go/src/net/http/client.go:189 +0x14
net/http.(*Client).do(0x0, 0xc00064e700)
        /usr/local/go/src/net/http/client.go:600 +0x225
net/http.(*Client).Do(...)
        /usr/local/go/src/net/http/client.go:582
github.com/dagger/dagger/engine/cache.(*client).GetConfig(0xc000109ab8, {0x1ca7af8?, 0xc0000b6a00}, {{0xc0002b2fe0?, 0x1cb2780?}})
        /app/engine/cache/service.go:261 +0x3f3
github.com/dagger/dagger/engine/cache.NewManager({0x1ca7af8?, 0xc0000b6a00}, {{0x1cb2780, 0xc000012520}, {0x1ca8930, 0xc000012530}, {0x1cb5c00, 0xc000618160}, 0xc000312a40, {0xc000046026, ...}, ...})
        /app/engine/cache/manager.go:73 +0x351

Signed-off-by: Marcos Lilljedahl marcosnils@gmail.com

[marcosnils]

I'm refactoring this so no need to 👀 yet