deps: vendor genqlient CLI

[bendrucker]

Fixes unexpected go.sum diffs by vendoring genqlient's top level package, which provides its CLI. Previously, only the underlying Go package (github.com/Khan/genqlient/graphql) was actually import-ed in a Go file, so the CLI-providing package didn't get considered for go.sum. By including -mod=mod, we had explicitly directed Go to not try to load this package from the vendor/ dir (which would fail) and instead use the normal implicit module downloading. When this happened, Go would record downloaded dependencies in go.sum.

In dependency update PRs like #70, this caused unexpected diffs:

diff --git a/go.sum b/go.sum
index f849e87..a66a6ce 100644
--- a/go.sum
+++ b/go.sum
@@ -14,6 +14,12 @@ github.com/acomagu/bufpipe v1.0.4 h1:e3H4WUzM3npvo5uv95QuJM3cQspFNtFBzvJ2oNjKIDQ
 github.com/agext/levenshtein v1.2.3 h1:YB2fHEn0UJagG8T1rrWknE3ZQzWM06O8AMAatNn7lmo=
 github.com/agext/levenshtein v1.2.3/go.mod h1:JEDfjyjHDjOF/1e4FlBE/PkbqA9OfWu2ki2W0IB5558=
 github.com/agnivade/levenshtein v1.0.1/go.mod h1:CURSv5d9Uaml+FovSIICkLbAUZ9S4RqaHDIsdSBg7lM=
+github.com/agnivade/levenshtein v1.1.1 h1:QY8M92nrzkmr798gCo3kmMyqXFzdQVpxLlGPRBij0P8=
+github.com/agnivade/levenshtein v1.1.1/go.mod h1:veldBMzWxcCG2ZvUTKD2kJNRdCk5hVbJomOvKkmgYbo=
+github.com/alexflint/go-arg v1.4.2 h1:lDWZAXxpAnZUq4qwb86p/3rIJJ2Li81EoMbTMujhVa0=
+github.com/alexflint/go-arg v1.4.2/go.mod h1:9iRbDxne7LcR/GSvEr7ma++GLpdIU1zrghf2y2768kM=
+github.com/alexflint/go-scalar v1.0.0 h1:NGupf1XV/Xb04wXskDFzS0KWOLH632W/EO4fAFi+A70=
+github.com/alexflint/go-scalar v1.0.0/go.mod h1:GpHzbCOZXEKMEcygYQ5n/aa4Aq84zbxjy3MxYW0gjYw=
 github.com/andreyvit/diff v0.0.0-20170406064948-c7f18ee00883 h1:bvNMNQO63//z+xN
gfBlViaCIJKLlCJ6/fmUseuG0wVQ=
 github.com/andreyvit/diff v0.0.0-20170406064948-c7f18ee00883/go.mod h1:rCTlJbsF
o29Kk6CurOXKm700vrz8f0KW0JNfpkRJY/8=
 github.com/apparentlymart/go-textseg/v12 v12.0.0/go.mod h1:S/4uRK2UtaQttw1GenVJ
EynmyUenKwP++x/+DdGV/Ec=