gvpr on Windows does not support absolute paths
## Steps to reproduce
Unzip e.g. https://www2.graphviz.org/Packages/stable/windows/10/msbuild/Release/Win32/graphviz-2.44.1-win32.zip and run gvpr with an absolute path:
Run:
1. `echo BEGIN{} > test.gvpr`
2. `Graphviz\bin\gvpr.exe -f C:"\Users\magja\Downloads\graphviz-2.44.1-msbuild-win32-release"\test.gvpr` (or whatever)
## Expected Behaviour
No error.
## Actual Behaviour
```
Graphviz\bin\gvpr.exe: Could not find file "C:\Users\magja\Downloads\graphviz-2.44.1-msbuild-win32-release\test.gvpr" in GVPRPATH
Usage: Graphviz\bin\gvpr.exe [-o <ofile>] [-a <args>] ([-f <prog>] | 'prog') [files]
-c - use source graph for output
-f <pfile> - find program in file <pfile>
-i - create node induced subgraph
-a <args> - string arguments available as ARGV[0..]
-o <ofile> - write output to <ofile>; stdout by default
-n - no read-ahead of input graphs
-q - turn off warning messages
-V - print version info
-? - print usage info
If no files are specified, stdin is used
The file does exist:
```
C:\Users\magja\Downloads\graphviz-2.44.1-msbuild-win32-release>more C:"\Users\magja\Downloads\graphviz-2.44.1-msbuild-win32-release"\test.gvpr
BEGIN{}
```
## OS Version
Windows 10
## Graphviz Version
gvpr version 2.44.1 (20200629.0800)
## Additional info
Relative paths work:
```
C:\Users\magja\Downloads\graphviz-2.44.1-msbuild-win32-release>Graphviz\bin\gvpr.exe -f test.gvpr
```
produces no error.
issue