Auto merge of #14850 - UK992:appveyor-ccache, r=larsbergstrom

Appveyor: Use ccache on Windows GNU builds

r? @larsbergstrom

Locally tested:

First run: (Build time: 26:27)
```
cache directory                     C:/github/servo/.servo/.ccache
primary config                      C:/github/servo/.servo/.ccache/ccache.conf
secondary config      (readonly)    C:/building/msys64/mingw64/etc/ccache.conf
cache hit (direct)                     0
cache hit (preprocessed)               0
cache miss                           953
called for link                        5
called for preprocessing               9
compile failed                         6
preprocessor error                    30
unsupported source language            1
autoconf compile/link                 43
no input file                          6
files in cache                      2523
cache size                         443.0 MB
max cache size                       5.0 GB
```

Second run: (Build time: 15:30)

```
cache directory                     C:/github/servo/.servo/.ccache
primary config                      C:/github/servo/.servo/.ccache/ccache.conf
secondary config      (readonly)    C:/building/msys64/mingw64/etc/ccache.conf
cache hit (direct)                   922
cache hit (preprocessed)              31
cache miss                           953
called for link                       10
called for preprocessing              18
compile failed                        12
preprocessor error                    60
unsupported source language            2
autoconf compile/link                 86
no input file                         12
files in cache                      2523
cache size                         443.0 MB
max cache size                       5.0 GB

```

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