Use more specific names for coverage variables to avoid clashes (#299)

We see a couple of cases in the main plz repo where we seem to be
picking up existing variables named things like `goCoverageFile`; I can
only assume there is some kind of prefix search happening in 1.23.

This makes them `_plz_goCover` which I think we can reasonably assume no
sensible Go program is going to use.

tools/please_go/ChangeLog

@@ -1,3 +1,7 @@
+Version 1.15.1
+--------------
+    * Use more specific names for coverage variables to avoid clashes
+
 Version 1.15.0
 --------------
     * Add support for Go 1.23 coverage generation (#294)

tools/please_go/VERSION

@@ -1 +1 @@
-1.15.0
+1.15.1

tools/please_go/cover/cover.go

@@ -44,9 +44,9 @@ func WriteCoverage(goTool, coverTool, covercfg, output, pkg string, srcs []strin
 	}
 	var cmd *exec.Cmd
 	if coverTool != "" {
-		cmd = exec.Command(coverTool, append([]string{"-mode=set", "-var=goCover", "-pkgcfg", pkgConfigFile, "-outfilelist", output}, srcs...)...)
+		cmd = exec.Command(coverTool, append([]string{"-mode=set", "-var=_plz_goCover", "-pkgcfg", pkgConfigFile, "-outfilelist", output}, srcs...)...)
 	} else {
-		cmd = exec.Command(goTool, append([]string{"tool", "cover", "-mode=set", "-var=goCover", "-pkgcfg", pkgConfigFile, "-outfilelist", output}, srcs...)...)
+		cmd = exec.Command(goTool, append([]string{"tool", "cover", "-mode=set", "-var=_plz_goCover", "-pkgcfg", pkgConfigFile, "-outfilelist", output}, srcs...)...)
 	}
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr