fix: gc policy for windows to use 20% of disk space

[profnandaa]

Initially we had the GC Policy for Windows use only 2 GB (2e9 bytes) of disk space and this was limiting for some build scenarios that need more than that, especially ServerCore images.

This commit makes the policy to use percentages as it is on Linux. Also going for 20%, double for that on Linux, since Windows images are generally larger. For instance, after the change, on a dev machine, the configured space was 74694367232 / (1 << 30) = 69.5 GiB.

fixes #4858 docker/buildx#2411

---

Before:

> main.getGCPolicy() C:/dev/container-core/buildkit/cmd/buildkitd/main.go:875 (hits goroutine(1):1 total:1) (PC: 0x1e1347e)
   870:                         All:          rule.All,
   871:                         KeepBytes:    rule.KeepBytes.AsBytes(root),
   872:                         KeepDuration: rule.KeepDuration.Duration,
   873:                 })
   874:         }
=> 875:         return out
   876: }
   877:
   878: func getBuildkitVersion() client.BuildkitVersion {
   879:         return client.BuildkitVersion{
   880:                 Package:  version.Package,
(dlv) p out
[]github.com/moby/buildkit/client.PruneInfo len: 4, cap: 4, [
        {
                Filter: []string len: 1, cap: 1, [
                        "type==source.local,type==exec.cachemount,type==source.git.checko...+2 more",
                ],
                All: false,
                KeepDuration: 172800000000000,
                KeepBytes: 512000000,},
        {
                Filter: []string len: 0, cap: 0, nil,
                All: false,
                KeepDuration: 5184000000000000,
                KeepBytes: 2000000000,},
        {
                Filter: []string len: 0, cap: 0, nil,
                All: false,
                KeepDuration: 0,
                KeepBytes: 2000000000,},
        {
                Filter: []string len: 0, cap: 0, nil,
                All: true,
                KeepDuration: 0,
                KeepBytes: 2000000000,},
]

After:

> main.getGCPolicy() C:/dev/container-core/buildkit/cmd/buildkitd/main.go:875 (hits goroutine(1):1 total:1) (PC: 0x2c63a5e)
   870:                         All:          rule.All,
   871:                         KeepBytes:    rule.KeepBytes.AsBytes(root),
   872:                         KeepDuration: rule.KeepDuration.Duration,
   873:                 })
   874:         }
=> 875:         return out
   876: }
   877:
   878: func getBuildkitVersion() client.BuildkitVersion {
   879:         return client.BuildkitVersion{
   880:                 Package:  version.Package,
(dlv) p out
[]github.com/moby/buildkit/client.PruneInfo len: 4, cap: 4, [
        {
                Filter: []string len: 1, cap: 1, [
                        "type==source.local,type==exec.cachemount,type==source.git.checko...+2 more",
                ],
                All: false,
                KeepDuration: 172800000000000,
                KeepBytes: 512000000,},
        {
                Filter: []string len: 0, cap: 0, nil,
                All: false,
                KeepDuration: 5184000000000000,
                KeepBytes: 140000000000,},
        {
                Filter: []string len: 0, cap: 0, nil,
                All: false,
                KeepDuration: 0,
                KeepBytes: 140000000000,},
        {
                Filter: []string len: 0, cap: 0, nil,
                All: true,
                KeepDuration: 0,
                KeepBytes: 140000000000,},
]